Which musl issues are to do with GCC? Alternate C libraries are common on Linux e.g. uclibc, dietlibc, bionic... Not to mention also the other OSs GCC runs on that don't use glibc. Of course, mixing C libraries between the main executable and libraries probably won't work.
As there are common people on forums searching for problems with their C code crashing and burning, because while those libraries conform to ISO C standard, their implementation defined semantics aren't the same.