As far as I know, at least the common ones map the same on all three platforms. There might be some weird ones that don't map (to my knowledge, they all map, but I'm not a huge shortcut user).
Gimp in OSX, last I knew, was sort of sandboxed in X11 and not integrated into the Mac interface, so everything maps like a non-Mac computer (and you have to use CTRL instead of CMD). I think they're working on changing that, so I don't know how long this will be the case. I suspect that once it's fully integrated, it'll be the usual CTRL-CMD conversion.
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Gimp in OSX, last I knew, was sort of sandboxed in X11 and not integrated into the Mac interface, so everything maps like a non-Mac computer (and you have to use CTRL instead of CMD). I think they're working on changing that, so I don't know how long this will be the case. I suspect that once it's fully integrated, it'll be the usual CTRL-CMD conversion.