Not sure what I need to do. Apple online support suggested that I just tick the box next to the XGimp icon in Extensions on my Mac. Did that and still cannot access the program.
Not sure who makes XGimp, but fairly sure the official Gimp project has nothing to do with it. So you’re unlikely to get help with that here. Also, it seems to be intended for iPads, so why are you trying to install it on macOS? And the fact that it tries to install a Safari extension is not a good sign, when it has no business being there. I would stay far away from it.
And the first review on the App Store:
Just get the real Gimp from here:
I concure — good advice.