if it's a serious hobby for you, a photoshop subscription is worth it. learning gimp doesn't get you ahead in the industry. neither programs are at all intuitive, but photoshop is dominate so it is easier to find tutorials on different techniques. and it's set up very well for professionally printing photos.
also if you learn photoshop, learning other adobe programs will be a bit easier. knowing how to use creative suite is an employable skill.
if you want to stick with gimp, pick one manipulation you are trying to accomplish and master it. then move on to the next. i can't remember it exactly, but it will do some things like very precise photo resizing, exporting in a variety of formats, detailed level and curve adjustment, switching between rgb and cmyk -- some of the things you really need. i can't remember if it has a nice unsharp mask, but you'll need that.