hiya gp main here, general rules:
1 barrels love bushes most people can't react fast enough to a barrel from a bush to them if your fast enough.
2 try putting and keeping barrels outside of enemy vision like over a wall or ofcoarse the barrels natural habitat the bush. they can't kill what they can't see also pray you don't fight any ranged champs
lastly 3 gp is basically a melee adc with some tank potential kinda (i say kinda cuz people build him 50% damage 50% tank but full damage works just was well if you need it but you will need damage) so your ult will not be ment for executing but more for assisting so unless someone is less than 5% hp it probably wont kill unless you go full ap (which i never suggest)
build:
triforce always triforce its wonderful on gp even before the rework since q procts it and barrels not only activate sheen but they also pass the damage along this is a good reason why they hurt
infinity edge, yes i'm aware of the marksmen changes it's still a staple for crit users 250% crit damage with sheen no wonder he one shots squishys
phantom dancer it's good...like its gonna be broken that's how good
from here it depends on what you need for example new essence reaver and hydra with boots are what i run while you could run spirit visage and randuins omen if you wanna tank a bit
ult: basically the only ones worth getting are fire at will 
and raise moral 
in that order! always first 500 serpents fire at will deaths daughter is best chained with some cc but on its own the single shot isn't worth it
overall stuff: laneing with gp is farm focused and holding control over your lane people who are very aggressive and can't back it up will feed but then there's the ones who can back it up like darius and garen they have the damage to bully and the bulk to out damage you in the early game.
watch your opponent try to lead them into barrel radius and most importantly farm with q and combo barrels and gunshot for wave clear.
grouping with gp can be strong even without barrels granted you got damage to back it up and you got a decent team
tl;dr he's susceptible to being bullied but will thrive in the late game with crit, triforce helps gp no matter your build, barrels are risk reward so if you can be sneaky or skillful it's to easy to kill people with a single barrel gunshot combo
then went Frozen Heart/Merc - Spirit Visage - Tri-Force - Dead Mans Plate.