Oh sure it's doable, and that's a potential reason to do it, cross-compatibility, it's just not compatible with the way RP handles palettes and would require significant changes. It would make more sense to simply have a separate converter for turning Gimp palettes into the format used by RP, operating on the text file itself. Someone could do this pretty easily.
Or a simpler approach would just be if you drop a .gpl into the normal palettes directory, REXPaint can just detect and automatically convert the first 192 colors for you and delete the original. Basically a built-in importing feature. That would be quite easy, too.
2.0 is going to be a who-knows-how-long kind of wait xD