You should be able to see how many barrels GP has

It doesn't need to be as obvious as Annie passive, but can be something like a buff icon with a number on it, at status bar when you click on him, just like how you are able to see Kayle's E duration and attack her when it's on cooldown, I want to be able to use the same strategy against GP

or give it a mana cost. He can keep 4 at once idc, if that costs some mana.