Awesome first finished map! Even a great map was it not your first one.

I have to commend you for getting this far on your first GIMP rodeo. Mine was not nearly as good. It took me a bit get a hang of it. I still use GIMP. It's a great program.

The mountains are done solidly, the trees as well. I agree with the others, the variation in colors added a nice pop.
Feedback wise, I agree that grass being that big relative to the trees is odd, but done right it wouldn't be a problem. I have loads of maps where if the overview maps were to scale, my trees would be gigantic. So, it's a balance of getting your lineart to fit the style.

All in all, very nicely done. looking forward to seeing your next.