I can't claim to know anything about GCC's internals, but from what I know/read, it does have a plugin system and I don't see any indication of that being held back for political reasons (there are many other potential reasons for technical inferiority).
And I don't see how the FSF could ever have furthered their political goals by making it technically hard to use GCC in free software tools, quite the contrary. Would be quite interested in evidence of that.