Its such a shitty backronym. All because someone had watched pulp fiction and decided it was funny.
Gimp is never going to spread as much as it needs to because of this name. Imagine telling your mother that she should go download "gimp" without feeling embarrassed.
I don't think the name is why it isn't used so much. I think it's because it's un-intuitive and almost incomparable to non-destructive/parametric professional programs. You can't even change text after applying a style... for many, things like this makes it practically useless for real work.
Someone already tried to fork GIMP in order to change the name (Glimpse)... it didn't work and they gave up.