To clarify, the _compiler_ is GPL (as is GCC, GPL doesn't mean that artefacts created with it share that license), but the standard library is the boost software license (which is not a copyleft library) so any machine code artefacts can remain private and doesn't require attribution.
[0] The NESFab standard library (lib/), examples (examples/), and documentation (doc/) are licensed under the Boost Software License 1.0 (See lib/LICENSE_1_0.txt).
While software made using this SDK can be any license, as stated in the readme:
>TL;DR: If you make a game using NESFab, your code can remain private, and you do not need to include attribution in your binary release.