Taking a look at your gallery I could debate with you on the "not that good" part. You've done some very nice work. I've seen several artists that get amazing results out of Gimp so an expensive 2D editor might not be the solution rather than getting a better understanding of the one you have. Are there any specific postwork techniques you want to learn it might be good to ask on a board where there are more experienced Gimp users.

One product I can reccomend for D|S artists wanting to enhance their postwork is Dreamlight's Surface Mask Creator. It really opened my mind to postwork and what could be achieved. This product has some decent documentation on how to set up and use masks in Gimp too.

http://www.daz3d.com/i/3d-models/-/surface-mask-creator?item=5636&_m=d

Learning how to manipulate layers and blending modes is also important and can yield quick and amazing results. Practicing and testing is absolutely priceless.