Short answer: yes.

Long(er) answer: you can do in GIMP almost anything that you can do in Photoshop. You have to adapt your workflow though... GIMP and PS work a little different in a number of things. So if you are following tutorials, you can't always just copy the steps from PS to GIMP.
On the other hand, people have claimed that in some ways GIMP offers even more options than PS. (I wouldn't know: I don't have PS)

If you browse this forum carefully, you with find tons of (beautiful, artistic, perfect) maps that were made with free programms like GIMP, Inkscape... or even MS Paint!
One of my favorite tutorials for making maps in GIMP is Gidde's Hand Drawn Maps tutorial
You can achive amazing result with relatively minor effort.
For other styles, both Tear's Saderan tutorial, or Ascension's Atlas Style can be adapted to GIMP.