Well the GPL specificly isn't a problem here, however it is really nice to have an alternative to GCC that actually encourages and facilitates reuse of their code rather than one that puts up deliberate obsticles to reuse even by other free software projects.
One of the big reasons that CLang was created was because there were some free software developers that wanted to integrate high quality front-ends (parser, etc) into other projects like IDEs, LLVM, and such. They prefered to work together with GCC to share the effort, but GCC refused. They were so paranoid about proprietary applications using GCC code that they refused to seperate the front-end into a GPL library that GPL applications could use. Their rational was that someone could easily write a GPL wrapper application around that GPL library that just serialized the data to/from a text representation, which could then be legally used with a proprietary application. In their mind, it was more important to make it difficult for proprietary applications to use their code than to make it easy for free software to use their code.
So LLVM was forced with the decision to either fork GCC or write their own. GCC was never designed with front-end modularity in mind, and a lot of changes would be necessary to do so. Once that massive refactoring was complete, it would be difficult to share improvements between the two codebases. Between that and some compelling technical reasons they chose go write their own and CLang was born.