Its basically like Photoshop. Has some nice features too it. If you go to this site: http://www.gimptalk.com/ They give you probably the best Gimp Signature Making tutorials and on the planet renders forum they have some there too. Its all about practice, I can do some ok stuff on Gimp but I am considering switching over to PS because its my 4th year on gimp and I want to get into the more advanced and realistic stuff.
And just search Gimp on Google and you can find a download for it.