Sure, but what about the inverse? With LLVM you can use frontends as libraries for integrating into an (possibly non-free) IDE for completion, syntax highlighting, refactoring etc. This is especially powerful because you can avoid serialization and just keep the ASTs in memory. I'm not aware of this being possible with GCC, but I could be wrong.
http://gcc.gnu.org/onlinedocs/gccint/Plugins.html
The wiki has links to some plugins too http://gcc.gnu.org/wiki/plugins