diff --git a/conanfile.py b/conanfile.py
index 1ca432b6..dd08f782 100644
--- a/conanfile.py
+++ b/conanfile.py
@@ -47,7 +47,7 @@ class UnitsConan(ConanFile):
     settings = "compiler", "build_type"
     requires = (
         "fmt/7.0.3",
-        "ms-gsl/3.1.0"
+        "gsl-lite/0.37.0"
     )
     options = {
         "downcast_mode": ["off", "on", "auto"],
@@ -135,7 +135,7 @@ class UnitsConan(ConanFile):
         self.cpp_info.names["cmake_find_package"] = "mp"
         self.cpp_info.names["cmake_find_package_multi"] = "mp"
         self.cpp_info.components["units"].name = "units"
-        self.cpp_info.components["units"].requires = ["fmt::fmt", "ms-gsl::ms-gsl"]
+        self.cpp_info.components["units"].requires = ["fmt::fmt", "gsl::gsl-lite"]
 
         compiler = self.settings.compiler
         version = Version(self.settings.compiler.version)
diff --git a/docs/usage.rst b/docs/usage.rst
index ca034f91..47690188 100644
--- a/docs/usage.rst
+++ b/docs/usage.rst
@@ -18,7 +18,7 @@ This repository contains three independent CMake-based projects:
     but until then it depends on:
 
     - `{fmt} <https://github.com/fmtlib/fmt>`_ to provide text formatting of quantities.
-    - `ms-gsl <https://github.com/microsoft/GSL>`_ to verify runtime contracts with ``Expects`` macro.
+    - `gsl-lite <https://github.com/gsl-lite/gsl-lite>`_ to verify runtime contracts with ``Expects`` macro.
 
 - *.*
 
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index eabb140d..74f20975 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -34,7 +34,7 @@ set_property(CACHE UNITS_DOWNCAST_MODE PROPERTY STRINGS AUTO ON OFF)
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")
 
 find_package(fmt CONFIG REQUIRED)
-find_package(Microsoft.GSL CONFIG REQUIRED)
+find_package(gsl-lite CONFIG REQUIRED)
 
 # library definition
 add_library(mp-units INTERFACE)
@@ -42,7 +42,7 @@ target_compile_features(mp-units INTERFACE cxx_std_20)
 target_link_libraries(mp-units
     INTERFACE
         fmt::fmt
-        Microsoft.GSL::GSL
+        gsl::gsl-lite
 )
 target_include_directories(mp-units
     INTERFACE
diff --git a/src/include/units/bits/constexpr_math.h b/src/include/units/bits/constexpr_math.h
index 53e4fdf4..d57ec497 100644
--- a/src/include/units/bits/constexpr_math.h
+++ b/src/include/units/bits/constexpr_math.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <gsl/gsl_assert>
+#include <gsl/gsl-lite.hpp>
 #include <units/bits/math_concepts.h>
 #include <units/bits/pow.h>
 #include <units/bits/ratio_maths.h>
@@ -65,7 +65,7 @@ struct decimal_fp {
  */
 [[nodiscard]] constexpr double constexpr_log(double v) noexcept
 {
-  Expects(v > 0);
+  gsl_Expects(v > 0);
 
   // lookup table to speed up convergence for all significant values
   // significant values of 7 and greater benefit mostly as they now converge in 5 terms compared to O(10)-O(100)
diff --git a/src/include/units/bits/ratio_maths.h b/src/include/units/bits/ratio_maths.h
index 31a188e7..a679c6bb 100644
--- a/src/include/units/bits/ratio_maths.h
+++ b/src/include/units/bits/ratio_maths.h
@@ -30,7 +30,7 @@
 #include <numeric>
 #include <tuple>
 #include <type_traits>
-#include <gsl/gsl_assert>
+#include <gsl/gsl-lite.hpp>
 
 namespace units::detail {
 
@@ -183,10 +183,10 @@ constexpr void normalize(std::intmax_t& num, std::intmax_t& den, std::intmax_t&
   const std::intmax_t b0 = detail::abs(rhs) % c;
   const std::intmax_t b1 = detail::abs(rhs) / c;
 
-  Expects(a1 == 0 || b1 == 0);                               // overflow in multiplication
-  Expects(a0 * b1 + b0 * a1 < (c >> 1));                     // overflow in multiplication
-  Expects(b0 * a0 <= INTMAX_MAX);                            // overflow in multiplication
-  Expects((a0 * b1 + b0 * a1) * c <= INTMAX_MAX - b0 * a0);  // overflow in multiplication
+  gsl_Expects(a1 == 0 || b1 == 0);                               // overflow in multiplication
+  gsl_Expects(a0 * b1 + b0 * a1 < (c >> 1));                     // overflow in multiplication
+  gsl_Expects(b0 * a0 <= INTMAX_MAX);                            // overflow in multiplication
+  gsl_Expects((a0 * b1 + b0 * a1) * c <= INTMAX_MAX - b0 * a0);  // overflow in multiplication
 
   return lhs * rhs;
 }
diff --git a/src/include/units/bits/root.h b/src/include/units/bits/root.h
index 6ae24383..e718ef8a 100644
--- a/src/include/units/bits/root.h
+++ b/src/include/units/bits/root.h
@@ -22,7 +22,7 @@
 
 #pragma once
 
-#include <gsl/gsl_assert>
+#include <gsl/gsl-lite.hpp>
 #include <units/bits/constexpr_math.h>
 #include <units/bits/math_concepts.h>
 #include <units/bits/pow.h>
@@ -38,7 +38,7 @@ template<std::intmax_t N, typename F>
   if constexpr (N == 1) {
     return v;
   } else {
-    Expects(v >= 0);
+    gsl_Expects(v >= 0);
     if (v == 0) {
       return 0;
     }
diff --git a/src/include/units/one_rep.h b/src/include/units/one_rep.h
index 18d19df3..4e2573b9 100644
--- a/src/include/units/one_rep.h
+++ b/src/include/units/one_rep.h
@@ -28,6 +28,8 @@
 
 namespace units {
 
+struct invalid_one_rep {};
+
 /**
  * @brief A representation type to be used for unit constants
  *
@@ -88,11 +90,22 @@ struct one_rep {
 
   [[nodiscard]] bool operator==(const one_rep&) const = default;
   [[nodiscard]] auto operator<=>(const one_rep&) const = default;
+
+  [[nodiscard]] constexpr bool operator==(const invalid_one_rep&) const { return false; }
+};
+
+template<>
+struct quantity_values<one_rep> {
+  static constexpr invalid_one_rep zero() noexcept { return invalid_one_rep{}; }
+  static constexpr one_rep one() noexcept { return one_rep{}; }
+  static constexpr one_rep min() noexcept { return one(); }
+  static constexpr one_rep max() noexcept { return one(); }
 };
 
 }  // namespace units
 
 namespace std {
+
 template<>
 struct common_type<units::one_rep, units::one_rep> {
   using type = units::one_rep;
diff --git a/src/include/units/quantity.h b/src/include/units/quantity.h
index a3b91944..ffc05be5 100644
--- a/src/include/units/quantity.h
+++ b/src/include/units/quantity.h
@@ -270,7 +270,7 @@ public:
           invoke_result_convertible_to_<rep, std::divides<>, rep, const Value&>
   [[nodiscard]] friend constexpr Quantity auto operator/(const quantity& q, const Value& v)
   {
-    // Expects(v != zero().count());
+    gsl_ExpectsAudit(v != quantity_values<Value>::zero());
     using ret = quantity<D, U, std::invoke_result_t<std::divides<>, rep, Value>>;
     return ret(q.count() / v);
   }
@@ -280,7 +280,7 @@ public:
           invoke_result_convertible_to_<rep, std::divides<>, const Value&, rep>
   [[nodiscard]] friend constexpr Quantity auto operator/(const Value& v, const quantity& q)
   {
-    // Expects(q.count() != zero().count());
+    gsl_ExpectsAudit(q.count() != quantity_values<rep>::zero());
     using dim = dim_invert<D>;
     using ret_unit = downcast_unit<dim, inverse(U::ratio)>;
     using ret = quantity<dim, ret_unit, std::invoke_result_t<std::divides<>, Value, rep>>;
@@ -354,7 +354,7 @@ template<typename D1, typename U1, typename Rep1, typename D2, typename U2, type
   requires quantity_value_for_<std::divides<>, Rep1, Rep2>
 [[nodiscard]] constexpr Quantity auto operator/(const quantity<D1, U1, Rep1>& lhs, const quantity<D2, U2, Rep2>& rhs)
 {
-  // Expects(rhs.count() != zero().count());
+  gsl_ExpectsAudit(rhs.count() != (quantity_values<Rep2>::zero()));
   using dim = dimension_divide<D1, D2>;
   using unit = downcast_unit<dim, (U1::ratio / dimension_unit<D1>::ratio) / (U2::ratio / dimension_unit<D2>::ratio) * dimension_unit<dim>::ratio>;
   using ret = quantity<dim, unit, std::invoke_result_t<std::divides<>, Rep1, Rep2>>;
diff --git a/src/include/units/ratio.h b/src/include/units/ratio.h
index 2af76f9b..7b1ea90b 100644
--- a/src/include/units/ratio.h
+++ b/src/include/units/ratio.h
@@ -31,7 +31,7 @@
 #include <numeric>
 #include <type_traits>
 #include <tuple>
-#include <gsl/gsl_assert>
+#include <gsl/gsl-lite.hpp>
 
 namespace units {
 
@@ -53,7 +53,7 @@ struct ratio {
 
   explicit constexpr ratio(std::intmax_t n, std::intmax_t d = 1, std::intmax_t e = 0): num(n), den(d), exp(e)
   {
-    Expects(den != 0);
+    gsl_Expects(den != 0);
     detail::normalize(num, den, exp);
   }
 
@@ -92,7 +92,7 @@ namespace detail {
 
 [[nodiscard]] constexpr auto make_exp_align(const ratio& r, std::intmax_t alignment)
 {
-  Expects(alignment > 0);
+  gsl_Expects(alignment > 0);
   const std::intmax_t rem = r.exp % alignment;
 
   if (rem == 0) {  // already aligned
diff --git a/src/include/units/symbol_text.h b/src/include/units/symbol_text.h
index f38ee6f2..2b774de3 100644
--- a/src/include/units/symbol_text.h
+++ b/src/include/units/symbol_text.h
@@ -24,14 +24,14 @@
 
 #include <units/bits/external/fixed_string.h>
 #include <units/bits/external/hacks.h>
-#include <gsl/gsl_assert>
+#include <gsl/gsl-lite.hpp>
 #include <compare>
 
 namespace units {
 
 namespace detail {
 
-constexpr void validate_ascii_char([[maybe_unused]] char c) noexcept { Expects((c & 0x80) == 0); }
+constexpr void validate_ascii_char([[maybe_unused]] char c) noexcept { gsl_Expects((c & 0x80) == 0); }
 
 template<std::size_t N>
 constexpr void validate_ascii_string([[maybe_unused]] const char (&s)[N + 1]) noexcept
diff --git a/src/mp-unitsConfig.cmake b/src/mp-unitsConfig.cmake
index e9ad2a72..9dcca1ab 100644
--- a/src/mp-unitsConfig.cmake
+++ b/src/mp-unitsConfig.cmake
@@ -22,6 +22,6 @@
 
 include(CMakeFindDependencyMacro)
 find_dependency(fmt)
-find_dependency(Microsoft.GSL)
+find_dependency(gsl-lite)
 
 include("${CMAKE_CURRENT_LIST_DIR}/mp-unitsTargets.cmake")