It was originally created by Microsoft and as Herb mentions "all our target platforms", so most likely it gets special treatment.
There is absolutely no special treatment afforded to the GSL by any of the target platforms. While we can't inspect MSVC's source code, both clang and GCC do not have any support or affordance for the GSL whatsoever and it would be very unusual to expect MSVC's source code to have some kind of affordance for this library.
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