GIMP's pretty bad at digital art I'm afraid.
For a while they were trying to attract people, beginning to include pressure sensitivity. Selling it was a "Free Photoshop".
Then one of the devs blew a gasket because it meant supporting wacom and ever since then they've held a firm stance on it being a "Image manipulation tool".
Krita would be your best bet, but there's also:
http://drawpile.net/ - Not as good, but it has network support.
http://mypaint.org/about/ - No idea whether this is good or not.
If you really do want to stick to gimp, I suggest looking at:
https://sourceforge.net/projects/gimpshop.mirror/