My 0.02 Euro-Cent: There is a minor problem with contributors being overly harsh/ borderline abusive on the mailing list. In my > 15 years with the project, I have only had that problem with one single person, and I have resolved that by never again touching the system that particular person is responsible for, also not for testing.
The _real_ problem is in bugzilla, mostly with abusive users complaining about the time it sometimes takes to fix bugs ("Why didn't you fix this? Are you stupid or what? That bug has been open for _weeks_!") or who will not understand that their program has an error, and insist on the compiler sanctioning their particular non-standard usage. On bugzilla, there is also a rather minor problem with contributors being overly harsh/borderline abusive, but that is also quite restrictive. If we talk about gcc becoming a more welcoming place, bugzilla is the place to start.