I was looking at a Breeze icon theme for Gimp and stumbled upon this G+ thread: https://plus.google.com/104737390849732945317/posts/3ptTH9CxqdZ?sfc=true I don't know if any progress has been made since then, but here is what I can say. I think Gimp is a major software and the lack of integration makes feel Breeze as incomplete. For what I know, it is possible to create custom themes for Gimp. 1) Edit > Preferences > Theme is the place where the user can select a custom theme. 2) Themes created for 2.8 won't work under 2.10, so maybe it would be a good idea to focus on the latter. 3) There are several themes for Gimp, with custom icons, that can be a good starting point at least to understand the logic. For instance: https://github.com/draekko/gimp-cc-themes Breeze is an amazing suite of themes, it would be nice if it could encompass Gimp.
GIMP 2.10 already includes a monochrome icon theme that doesn't look exactly like Breeze, but it's in the same vein. I don't think we're going to explicitly create a Breeze icon theme for GIMP, but of course if someone else wanted to and submit it to GIMP, that would be lovely. :)