I'm just trying to improve my poor C knowledge

Be careful what you wish for. :) I'm not sure studying large and complex C code bases is a good way to learn C. I started by reading The C Programming Language, aka K&R, from cover to cover.

Having slaved over many huge C code bases over the years, I can assure you they all rely heavily on static and dynamic code analysis tools to keep the code clean (e.g. Coverity, Valgrind, AddressSanitizer and many more). P5P is no different, as indicated by the EXTERNAL TOOLS FOR DEBUGGING PERL section at perlhack.

So, if you want to become a serious C coder, you must master these tools.

I'm just trying to improve my poor C knowledge.

You picked something that's probably a bit too advanced for that. It involves not just C, but knowledge of FFI and of Perl internals as well, both of which are complex systems.

Is [replacing malloc with GC_MALLOC] recommended or will it cause problems in the future not yet imaginable now?

The answer to the question depends on whether FFI or Perl can free the returned buffer.

I think the const in the return type tells FFI to copy the string and avoid freeing it. If so, then yes, you could use a different allocator.

I'm guessing that because the example would have problem as-is if FFI or Perl could free the buffer.