When I was in college I took a few traditional art classes, in traditional media via traditional teaching methods and conventional teachers. We didn't even consider computers for art in the 70's. We have greatly progressed, there are artist tools on every PC platform and photo quality printers on the cheap. In fact the design profession has become quite entrenched with Adobe products taking the lead. It has gotten so deep that the full blown software is coveted by beginners, learners and want to be's globally. Go ahead aspire to that! It's OK! But wait!
In the backrooms of the Linuxville chateau, behind the bookcases, down the stairs in the cave of wonders, I've have discovered it is about the tools. As a kid I stressed about Red Ball Jets tennis shoes or Keds with the magic wedge. If I couldn't run optimally, they both were the same. It is obvious that Photoshop and other Adobe creative products are the grail of today's artist. But if you strip away the well explained interface, there are tools common to all graphic programs. The tools themselves are standard equipment. Think about it, the programming required to draw a line or a bezier curve or do layering, is pretty much the same in every program. Most good graphics programs have a standard set of drawing and editing tools , a way of adding custom scripted functions and extra tools.
What's the real diff? I think it is in the output of the digital file to printers. If you are doing RGB, you can get away with a lot because inkjet printers are RGB already. If you are talking offset or printing that requires you to separate color layers as with CMYK printing, that is more involved. Photoshop has built-in tools for CMYK output, GIMP does not. CMYK only makes Photoshop better if you need CMYK. With RGB printing becoming quite the bomb, GIMP is RGB cool.
We are told GIMP has a bit of a learning curve. That's a big curve if you are already into the Photoshop interface and work flows, way smaller if you have no experience. What's the secret? Stop comparing and criticizing and watch some video tutorials. Watch them, you'll say, dag man, I can do that! Believe me, once you learn to use say, a bezier curve, you can use that tool in any program that has it. So, what's holding you back, a $brand-name$?