@snmiller, You absolutely can do those edits in GIMP, in fact, that’s what I used for the edit that I just posted here!
I don’t know of any specific how-to guides, I would have to know what you’re interested in learning. I don’t like to recommend a website or video tutorial generically, because there is apt to be a lot of inappropriate content. If I knew exactly what you were wanting to learn, I could probably find a suitable tutorial without too much trouble.
I have also written a few of my own, which can be found here: GIMP Tutorials.