The compilers of 30 years ago had incompatible behaviors between OSes, for instance C or C++ had a great deal of variation. And 30 years ago you had pre-OSX MacOS as well.
Now you can just choose 1 compiler such as GCC and it works the same on all platforms; even cross-compiling to a different CPU architecture works well and is far easier to setup (before it was a black art).
In what way does GCC play any role in cross platform GUIs whatsoever? Itβs a very small niche on Windows, macOS, iOS and Android. Visual C++, respectively LLVM/clang are much more dominant on those, and even then they are barely the right tools for GUI (maybe except Windows, but even then competing with .NET).
Now you can just choose 1 compiler such as GCC and it works the same on all platforms; even cross-compiling to a different CPU architecture works well and is far easier to setup (before it was a black art).