The mailing list for GTK+ is firstname.lastname@example.org. Unfortunately, it seem to be serverly underused and it is almost impossible to get discussions going there. Most discussions seem to take place at bugzilla or the (in)famous planet.gnome.org. But part of the problem also seem to be that most gtk developers are employed by a few companies, namely Red Hat and Imendio. When you work in the same office it becomes easier to communicate orally, than having to use a text medium.
I think it would be nice if there was more "flow" on the mailing list. I fear that important information is getting lost and it creates a divide between employed hackers and others.