If you are an experienced GP player most of the following information will not be new to you.

Mid or Solo lane is preferred. If not you have to lane with a support hero who doesn't need creep kills.
You have to farm like hell, and last hit with Parrrley if necessary. Play safe, and harass with Parrrley. Only go for obvious kills, and keep your Remove Scurvey OFF cooldown bacause you might need it in case they counter gank you. !Remenber; Remove Scurvey cancel's crowd control spells, such as slow, stun, suppress, etc!
Dont push too much, but be sure to get ALL the creep kills. The reason why pushing with Gangplank is not clever early game, is because it makes you more vulnerable to ganks and allow the enemy to farm safely at their tower.
Get blue buff as often, and early, as possible.
If solo lane you might wanna grab a ward the first time you back. For safety.
From Lvl 6 and up you have to keep an eye on the other lanes frequently. So you can get kills or assist/safe teammates with Cannon Barrage, while safely farming your lane. Obvious tip: If you're on voice chat with your teammates let them know when your ult is up so they can request it.

When you have purchased Sheen and Boots of Speed, and if you cant afford the Brawlers->Avarice, I can recommend getting an Elixir of Agility for temporary crit chance.
When laning phase is ending you should at least have farmed your Sheen, Ionian Boots of Lucidity, Avarice Blade and Vampiric Scepter.

Blue buff is almost a necessity from lvl 6-11. You Should only be getting blue If you don't have a jungler or support/ap hero who need it more than you.
The most efficient way of killing Blue buff is by popping Raise Morale, Initiate it with Parrrley and attack it twice to stack up your passive. Keep spamming "Q" on the lizard while killing the two smaller neutrals. Use Remove Scurvey if you have too and don't be afraid to use all your mana because you will quickly regain it once you've recovered the buff.

Red buff is a very nice addition to the passive and later Frozen Mallet. You are w/o doubt the one on your team who has priority concerning red buff.

Since you'll be going mid/solo you want to be the master of 1on1 hence the exhaust, ignite combo. you could go with ghost, exhaust instead because your passive is like a weak ignite.
I will greatly disapprove getting flash for GP since he don't have any crowd control spells which flash would be useful with. So flash would be for escaping or closing a distance only which exhaust does just as well.

I have a couple of late game tips for you as well.
ALWAYS remember to pop your ult in the start of a team fight (obviously hit as many enemies as possible).
I prefer purchasing Ghostblade over Phantom Dancer because of the active and because it gives you more attack damage, and The black cleaver is more than enough attack speed. If you prefer Phantom Dancer feel free to sell the Avarice blade and get a Phantom Dancer or start out with buying a Zeal instead of the Avarice.
If you get the Ghostblade I can recommend dragging it to the upper left corner of your inventory. That makes it easier to use the hotkey (1). !Remember to use the active when you initiate or chase!
Cannon Barrage only have 120/115/110 seconds cooldown, so dont be afraid to "waste" it on minions. If you see a well pushed lane just nuke them down, you will most likely earn a lot more gold than you would by using it on enemy heroes because you will mainly be getting assists that way.

Let me know if i made any grammatical Mistakes Please :)
Hope this was useful to you.
Have fun.