At the time Herb made that comment GSL was documented as supporting XCode 12.5.1/13.2.1, GCC 10/11, Clang 11/12, and Visual Studio 2019/2022 using both MSVC/LLVM [0]. Even if MSVC had special support for GSL I'm a bit more skeptical that such support would extend to XCode, GCC, and Clang.
reply[0]: https://github.com/microsoft/GSL/tree/99a29ce797c8337b8923f2...