File is imported into gzip from a non-GPL'd project, that gzip is GPL-licensed doesn't relicense the file.

File in question literally starts with a comment stating

   inflate.c -- Not copyrighted 1992 by Mark Adler
   version c10p1, 10 January 1993

They can license the changes that they've made to that file since importing as GPL though. The linux copy still includes that header, despite also clearly having GPL code in it as well.

Nintendo might have a "I didn't know any better because the header was unclear" argument that might limit their practical damages, but if we can only find this file with associated changes in a GPLed project it certainly looks like GPL infringement.

I mean if the file header says you can do what you want, it seems reasonable to do what you want. If you're relicensing, you should modify or remove that.

If it's had changes since Adler's 1993 copy, then it doesn't matter what Adler says.

Grab the original rather than taking code out of GPLed projects if you don't want GPLed code.