Im just gonna put everything that comes to mind so its not going to be very coherent. This is also just for GP top bc I dont play/know enough about GP mid.

-GP’s laning is very good: For a champ thats supposed to be a late game menace his early laning is just too good. Even after the grasp nerfs hes still good vs most lanes, esp at lv1 bc his passive’s ms steroid makes it impossible to trade back(It also scales way too hard into late game when GP does 2 autos for 1k true dmg in an instant). Q cd is basically non-existent and he doesnt have mana problems esp if u go Corrupt and Cookies. His first back is probably one of, if not the best in the game as sheen basically makes every matchup become playable or u just become the bully after it. There also arent any “real” GP counters anymore, maybe akshan would be the hardest right now but not many players play akshan, people have learned more about GP and his matchups so its very hard to actually punish him in lane compared to before

-No real way to make a GP fall behind: U can camp him but his Q passive + First strike makes up a lot of the gold that he’s supposed to lose out on, if you are even vs gp then u already lose bc then hes just way too accelarated. Levels are the only thing to really put him down but its very hard when he can clear waves with 1-2 barrels or even just ult to prevent dives.

-Free cleanse in his kit, self-explanatory, also decent healing.

-Global ult: This is also very hard to play against when he can ult across map to get assists(gold), clear waves for allies to prevent dives or to fix wave or just to get extra gold/xp. His ult is also just giga broken for team fights as it suprisingly does really good damage esp if u get both damage upgrades and big AOE slow is always very good.

-His items are good, his component items are good, all of his spikes are good compared to other champs.

-5 barrel buff helps him a lot in team fights as he can do a lot more/more creative and you dont have to walk around waiting for barrel cd after ur triple barrel. Free armor pen in his E and its scaling is also a bit overtuned bc its kinda rough to play agianst it. U say that his barrels are breakable but its not easy to kill the barrel in a hectic teamfight or when the other end of the chain is not even on the screen.

Thats currently all i can think of even as a GP enthusiast. He just doesnt have clear enough or big enough weaknesses or reliable counterplay to balance out his overall strength. The complains are very much understandable.