There is no section 15 or 16 in GPLv3, but I did find section 7 saying:
Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it; or
But that's an optional thing that you must add onto the GPLv3 license. I'll have to keep that in mind for the future.
That would explain why what I've read mentioned it's not guaranteed in GPLv3 (when comparing to MIT). I'll have to figure out what that notice would look like.