1. You cannot do everything with GIMP. For instance, there is no free transform tool in GIMP, and no even remote substitutions for it exist. In the roadmap it currently has a rather low priority.
It's also not possible to make non-destructive changes such as adjustment layers or layer effects. The only existing bunch of scripts for layer effects provide a similar UI, but you can't adjust settings after you applied an effect. So you have to redo things again and again. And that's just loss of time.
2. Photoshop isn't designed that way. It's designed to remove as many roadblock as possible and to unclutter the work of its users. So that they could focus on actual work. Hence your argument about "who did the work" simply doesn't apply unless for some obscure reason you were referring to extensive use of third-party actions. Which anyway isn't what everybody mostly does.
So let's be honest. Just because GIMP is free, and Photoshop is proprietary, GIMP-powered designs aren't true art by humans, and Photoshop-powered designs aren't computer generated art.
Similarly, GIMP doesn't provide all the features that Photoshop has. Some of these features are irrelevant for most users, but a lot of them are quite important and help you complete your work faster.
sudo apt-get build-dep gimp
will get you all dependencies
then you can fetch sources and do the usual
./configure && make && sudo make install