Gimp is a really good program, there is no doubt about that.
But it would be nice if people do not always ask for program that there are on other systems, but would sometimes deal with the programs that already exist. Wonderbrush is hard for me to understand because i have not enough expirience with working with layers, but this is where people could start with simple tutorials instead of asking for alternatives. I think Wonderbrush is worth a look and people who are familiar with layers would get ahead quickly.
There was a old port of gimp back in beos times, but it needs x11.