Well, the main thing i would say:
The color of the render/character doesnt match well with the green background so it doesnt blend in very well. Parts of the background look nice, but other parts are just plain green with no brushing or effects. My advise would be pick colors for the background that compliment the color of your focal, and make a little bit more use of brushes and effects.
Overall its not bad for the short amount of time you have been using gimp

You'll get better the more you practice.