What do you build on GP? He's one of my mains, he's my most played champ in my side and my 2nd most played on my main. Why I love him is he only really gets counterpicked by Panth, and you can build him either really tanky or with lots of damage (triforce and IE) depending on what your team needs.
I've even invented my own build for him, Bankplank. It's good when you're vs a fairly non-aggressive enemy top laner but against a strong ganking jungler.
You start with mana regen thingy (that goes in to philo stone), 1 ward and 6 HP pots, then build philo stone and heart of gold, and with the extra gold from your Q you end up so rich by late game
. I use this build in most of my games with him actually.
Yeah, 3 rune pages is enough to be going on with really, it's more important to buy more champs and the best runes than more rune pages. They are so expensive as well
For your runes, for AD it's better to have +AD runes and quints rather than armour pen. This is because most abilities on AD champs scale off AD, and extra AD makes last hitting easier, so overall it's just better.
Otherwise it looks ok
Just save up and buy those last runes when you get the required levels, and you'll be good to go!
We should play sometime, I can give you some tips if you like, I'm Smilerrr.