diff --git a/src/core/include/mp-units/bits/external/fixed_string.h b/src/core/include/mp-units/bits/external/fixed_string.h
index b15d214c..64e1172d 100644
--- a/src/core/include/mp-units/bits/external/fixed_string.h
+++ b/src/core/include/mp-units/bits/external/fixed_string.h
@@ -28,6 +28,7 @@
 #include <mp-units/compat_macros.h>  // IWYU pragma: keep
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
+#include <gsl/gsl-lite.hpp>
 #include <compare>
 #include <cstddef>
 #include <cstdlib>
diff --git a/src/core/include/mp-units/bits/ratio.h b/src/core/include/mp-units/bits/ratio.h
index b1b6d286..d7d020f1 100644
--- a/src/core/include/mp-units/bits/ratio.h
+++ b/src/core/include/mp-units/bits/ratio.h
@@ -22,11 +22,11 @@
 
 #pragma once
 
-#include <gsl/gsl-lite.hpp>
 #include <mp-units/bits/external/hacks.h>
 #include <mp-units/bits/module_macros.h>
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
+#include <gsl/gsl-lite.hpp>
 #include <compare>
 #include <cstdint>
 #include <numeric>
diff --git a/src/core/include/mp-units/dimension.h b/src/core/include/mp-units/dimension.h
index 0ddd22bf..bef17e10 100644
--- a/src/core/include/mp-units/dimension.h
+++ b/src/core/include/mp-units/dimension.h
@@ -31,6 +31,7 @@
 #include <mp-units/bits/text_tools.h>
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
+#include <gsl/gsl-lite.hpp>
 #include <array>
 #include <iterator>
 #include <string>
diff --git a/src/core/include/mp-units/quantity.h b/src/core/include/mp-units/quantity.h
index 270d27bd..720ee57d 100644
--- a/src/core/include/mp-units/quantity.h
+++ b/src/core/include/mp-units/quantity.h
@@ -35,6 +35,7 @@
 #include <mp-units/reference.h>
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
+#include <gsl/gsl-lite.hpp>
 #include <compare>
 #include <utility>
 #endif
diff --git a/src/core/include/mp-units/unit.h b/src/core/include/mp-units/unit.h
index 4526333e..7ef58e03 100644
--- a/src/core/include/mp-units/unit.h
+++ b/src/core/include/mp-units/unit.h
@@ -39,6 +39,7 @@
 #include <mp-units/bits/unit_concepts.h>
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
+#include <gsl/gsl-lite.hpp>
 #include <array>
 #include <iterator>
 #include <string>