Is there one, or is this it right here? 
The post counts on GimpTalk make it seem like a useful forum. There are sections devoted to other open source software (including Blender).
Try them.
I just came from GimpTalk - they’re slow as hell, apparently been under DDos attack for the last month or unable to fix the damage from being hacked.
If GimpTalk is the best out there, then ba.org is a better GIMP forum and should perhaps have a GIMP subforum.
Yes, GimpTalk is being a bit (a lot) on the slow side lately. Don’t give up on it because of that, though. If the warning at the top has some truth in it, it should be resolved soon.
And, well … I guess having a GIMP forum here wouldn’t hurt, but then again, it would only be necessary if there’s a sufficient amount of help requests specific to GIMP.
GimpTalk has completely stopped loading 
this a nice one http://gimp-tutorials.net/
and this one right here in blenderartists is also very good.
It is a shame that GimpTalk is having such a difficult time. That site has by far been the most responsive forum I’ve been on. I’ve occasionally found myself on GimpDome, although the emphasis there is more on competitions.
Although not a forum, I’ve found the video podcast, Meet the Gimp, to be helpful as well.
Well I was able to use Gimp Talk for a couple days, now I can’t log in. If this keeps up I’ll have to try one of the other sites.
Free software always lacks a lot of features that are in paid products.
In fact, Gimp is an example, the functionality of Adobe Photoshop is much broader.
Yes, and large corporations mostly buy software, since paid software always has reliable support.
So this choice is just a home user and that is rather mediocre …
If I already had a choice, then among the free software I would rather choose the editor Krita