Having never used GIMP, but being in the same boat at one point as yourself, I would say, if GIMP as some of the same, major features, such as Marque Masking (as in be a selection tool), layers (as in, the ability to lay one thing on top of the other), and the most important (in my option) adjustment features (Hue\Curves\Levels) or something similar. Then it should be easy. Photoshop is loaded with a lot of different features and “bells-and-whistles” that, do, for the most part, help in work flow, but for the most part, it all comes down to Layers, and adjustment features. So, I would suggest learning on GIMP if they have these features, then moving onto Photoshop. There items might be named differntly, and the interface is different, but for the most part, it would be like going from driving a manual truck, to an automatic economy car. In essence, everything is the same, they are just in different areas. I’ll go look at GIMP and come back with more details.