Gimp is cool because it's free, but if you don't mind spending a little bit to get a decent program, Affinity Photo is very comparable to Photoshop that doesn't even come close to breaking the bank (about $60 USD). There is also Affinity Designer (like Adobe Illustrator) for the same price, and a free beta of Affinity Publisher (like InDesign). Plus Photo and Designer are other available for the iPad and they are just as powerful as the desktop versions - something Adobe doesn't have.