The guidelines say here that GSL Expects always aborts on failure.
http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-assertions

However, Microsoft's GSL implementation has a mode that can alternatively throw on failure. see here microsoft/GSL#775 which prevents making some functions unconditionally noexcept.