It probably isn't you so much as GP himself. They nerfed him so many times and so hard that he isn't that good anymore. Before he used to have his barrels be poke and do lots of damage, now not only can he barely use barrels due to low limit, but he HAS to have them if he wants to outtrade.
But GP generally play similar to Nasus where you basically just farm first 10 minutes and basically ignore the opponent. Keep on eye on the map to see if your ult can get some assists or even kills on other lanes. As for his build it mainly comes to how good you are early game. If you somehow get ahead early(rare since GP is bad earlygame, very easy to kill) you can build a nice critplank early. I often always build the same items on him. Trinity, IE, Warmogs, Spirit Visage, Dead Mans, and Lucidity or Swifties. But as to which order again is how the earlygame goes. If I am winning and doing very well, I will build Trinity and offensively first, if I am neutral or losing, build tanky first.
This is just someone that used to play GP quite a bit when he got reworked til they nerfed him too much. I can understand he was a bit overbearing but they went too far I feel. Barrel nerfs alright, but the Q and ult nerfs? Q without refund is fked up and they basically made it take twice as long to make his Ult useful due to reducing Silver income.
