> that happens to also be an almost-unknown slang
Perhaps where you live.
Where I do, it's easily the most common slur for people with physical disabilities, and it's considered very offensive. I do consciously avoid using or talking about GIMP at the office, using it only when I'm working from home instead, specifically to avoid offending coworkers. This isn't me working hard at getting offended; it's me putting a minimum of effort into not upsetting my colleagues.
The folks who run GIMP are absolutely free to keep the name. But I think it would only be mature of them, and fans of the software, too, to be understanding of how others will perceive it, and not get defensive about the name adversely impacting the app's popularity.