GIMP is the defacto noncommercial photo editing program for Linux, Windows and Macintosh.
It is no more difficult to use than Photoshop to a new user.
Unfortunately, someone who has a large amount of experience with Photoshop would likely find transitioning to virtually any software which was functionally equivalent to Photoshop. They changed the names of many of the functions and that was probably due to a desire to avoid lawsuits. However, when I made the transition, I learned just one technique at a time as they were equivalent, but using different commands.
PS and GIMP can do most of the same things and they are very complex techniques.
There are books available to make the learning easier.
Larry
"Learn from the mistakes of others, You'll never live long enough to make them all yourself!"
(No trees were harmed in the sending of this message, however, a significant number of electrons were quite inconvenienced.)