Yeah, I play GP quite a lot.
For runes I go AD marks, Health per level seals, MR glyphs and AD quints. This can change depending on the lane match-up but for the most part I just go that.
Masteries I mostly go 12/0/18:
This is just to help you out in lane since laning phase is the only part of the game a GP should have trouble with. Grasp of the undying is the strongest keystone in-lane; it won't scale with GP but GP doesn't really need any scaling at all... Thunderlords won't benefit him and Deathfire Touch is too lategame to even be viable; all the other keystones are shit.
Don't bother CSing with your Q early game; there's no point and it's a really slow way of farming up for your ult upgrades, especially since the nerf where you don't get your mana back from killing things with your Q. Instead harass with your Q whenever grasp of the undying is up. Once you have trinity force and a BF sword, barrel Q should be able to clear minion waves, so that's the point where you start farming for your ult upgrades.
Build path: Start with corrupting potion (try to harass as much as possible with Q while you're drinking the potion) > If you have enough money for a sheen then buy a sheen, if not, buy components of phage > complete trinity force > BF sword > CDR boots > either turn that BF sword into an Infinity edge or an Essence Reaver (Either way, you're getting both IE and Essence Reaver but the order you buy them depends on the situation) > everything after this point is situational; I never buy any defensive items on GP unless it's Mercurial and majority of the time I seem to find myself buying a phantom dancer just for the extra crit. Lord Dominik's regard is also good since you shouldn't really be buying health on GP anyways, so the multiplier onto your damage is really nice.
Never. NEVER... buy stattik shiv. On ANYONE. It's shit, except for maybe Yasuo.