It was originally created by Microsoft and as Herb mentions "all our target platforms", so most likely it gets special treatment.
replyAt 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...