GIMP has five official mailing lists, hosted by lists.xcf.berkeley.edu (don’t try to access this website directly). Use these rather than their corresponding forums on www.gimpusers.com. Take a look at www.gimp.org/mail_lists.html first if you are not familiar with list etiquette.
GIMP User (https://mail.gnome.org/mailman/listinfo/gimp-user-list) is an active mailing list for GIMP users. Despite what the official site states, it is not especially Unix based. You can ask anything you want, but please first look at the documentation as well as the list archives. If your question is well formulated, you will get several useful answers.
GIMP Developer (https://mail.gnome.org/mailman/listinfo/gimp-developer-list) is also an active mailing list. Despite its name, this list is not used only by developers. It is aimed at them, however, so don’t use this list to ask a simple user question. Use it for suggestions you feel are of general interest or to prepare a bug report.
GIMP Web (https://mail.gnome.org/mailman/listinfo/gimp-web-list) deals with the GIMP website and has been somewhat dormant for a while.
GIMP Docs (https://mail.gnome.org/mailman/listinfo/gimp-docs-list) deals with GIMP documentation. The people who build and translate the documentation are the primary users, but you can use it to point to an error, for example.
Two IRC channels are available, called #gimp and #gimp-users. Avoid them if you are not accustomed to using IRC. They can be completely dormant sometimes and very active at other times.