try installing gmp

Author: brianlball

ci/github-actions/setup.sh

@@ -126,7 +126,16 @@ else
         ulimit -a
         ulimit -n 4096
         ulimit -a
-
+        
+        if [[ "${INTEL}" == "false" ]]; then
+            # Ensure gmp exists for native gems (bigdecimal link step uses -lgmp)
+            brew install gmp
+
+            # Help the portable ruby toolchain find ARM Homebrew libs/headers
+            export CPPFLAGS="${CPPFLAGS:-} -I/opt/homebrew/include"
+            export LDFLAGS="${LDFLAGS:-} -L/opt/homebrew/lib"
+            export PKG_CONFIG_PATH="${PKG_CONFIG_PATH:-}:/opt/homebrew/lib/pkgconfig"
+        fi
     elif [ "${ImageOS}" == "ubuntu22" ]; then
         echo "Setting up Ubuntu for unit tests and Rubocop"
         # install pipe viewer to throttle printing logs to screen (not a big deal in linux, but it is in osx)