I play alot of gp top as crit plank.
I start off with flash, ward and red pot. you won't do much dmg until 6 so farm safely.
I then build statikshiv, phage, boots, and the rest of triforce. Warmogs, and then the last item is up to you frozen mallet is good too.
as for boots, i would build the armor ones or merc trades.
I farm alot easily when i rush shiv and my ult is really handy in stealing dragons and assisting other ganks from other lanes. The cd on gp ult is not very long so you can use that to farm if your lane is pushed while youre at base with no tp.
Just make sure to get movement speed quints as they do help! <3