If anyone is still having trouble with Klepto GP, pick up Yorick
Trust me, guy absolutely dominates lane vs GP. He can viably sustain through GP's early Q harass and at level 3 with ghouls up you can all in him and blow his flash due to him having no other way to escape the wall. You can do this at any point to blow his flash and later when his flash is down he is just a free kill for you or the jungler.
Ever since I found this out I've been picking Yorick as a counter to him and haven't lost lane to a GP yet (like, not even close).
And by winning lane you just hold so much more map pressure especially as Yorick. And if you see noone is top to help GP out, you can just walk under tower and destroy it cuz GP can't even burst you down to stop you.
It's truly beautiful.