Stuff you'll spend hours fighting with in GIMP are literally 2 mouse click fixes in Affinity.
Obviously you could spend $0 and just use GIMP as well, but I personally find the $50 for Affinity Photo's amazing interface, speed, and live filters to be infinitely worth the investment. Affinity Photo is close enough functionally to Photoshop that most of the Photoshop tutorials should be pretty easy to follow along. You'll be much, much better off in the long run. Then go over to and go through as many tutorials as you have time for. If I was going to spend money on a program for digital mapping and didn't want to pay Photoshop's exorbitant subscription price, $50 for a copy of Affinity Photo will be the best money you can spend. I don't regret that time spent using the software for the most part, because it taught me a bunch of tricks I could later apply using actual image editors (Photoshop / Affinity Photo), but I would literally never use it again.
I spent a lot of time learning CC3 when I bought it way back in 2009, only to realize that the results you could get from it were wholly dependent on which of their tile sets / map style assets you had available.