LLVM exists because its original developers wanted to do research, not because of GPL and at the time there wasn't even a C compiler for it, not to mention that GCC wasn't designed at all to do what LLVM did.

The license angle with Apple did happen because of GCC's switch to GPLv3 but that came years later - and GCC still wasn't (and AFAIK still isn't) designed to be used like LLVM or (later) Clang, like using it as a backend for IDEs, analyzers, language servers, etc. The two projects aren't interchangeable with only the license being the thing that differentiates them.