GPI is given based on your streak (50 max) at the end of each workout, and then you are given 17 for each level you pass, and you get the GPI from passing levels only at the next workout complete.

GPI is rewarded quite a bit differently from toneGym, does this make sense?

makes sense but it's so strange. VERY different than SoundGym or ToneGym. Not really a fan if I'm being honest.

Here you earn GPI just for training daily, even if you don't beat any level.
I don't like it either.

I like some aspects of it, in that because TPI growth is exponential and GPI is not, you are rewarded the same amount for passing every level (as at ToneGym the best way to get TPI is to spend all your time on one game until you red star it and than start the next) instead here at Guitar gym you are encouraged to at least pass the easier first few levels of each game. IMHO I think a mix of the two would be best, encouraging practicing every game, but also rewarding passing the difficult higher levels.

GPI for every day you show up is fine, but I think it should be a lot less, as 365*50=18,250 and 2*365*50= 36,500 which is already well past Diamond. I have 23,868 just from games, and because I do no complete a workout most days My GPI looks quite small.

Daily workout GPI is either too high or the bar for highest level (diamond) is too low. It was a lot easier to obtain it here compared to for example SoundGym - if I remember correctly, there it took me like 1,5 years of subscription. But I am not like super bummed about that - guitar gains are far more important to me than some stats 😊.