To put a simple cap on this argument ( discussion ? ) is it really about Gimp vs Photoshop or more about Linux vs Windows - my views are as follows .
Adobe Photoshop is a lot more widely used for one simple reason it runs in the windows environment and that I believe is the only reason for its preferential choice . Photoshop is the finished article that you pay for , you can add things such as pluggins and play about with the settings to suit yourself .
How many normal pc users have actually seen, let alone tried any of the Linux Distros ? The Gimp is on a par with Corel Paintshop Pro X , with two major factors in its favour, one it is free and two it is open source, so if you dont like what it does and you can handle Linux command line programming quite well , you rewrite the code to make it do what you want it to do, failing that there are numerous Linux Communities who will willingly help you out with professional advice , free software and ready made command lines to get you going with whatever your trying to achieve .
Summary - Adobe Photoshop and Windows XP - 8/10 ( how much does this cost ? )
The Gimp and Linux - 7/10 ( and this is all free )