For the masteries you go with the standard 12 ferocity and 18 cunning with the thunderlords keystone. Starting with the ferocity, you definitely want sorcery over fury as as your build will give you more than enough attack speed and GP is more of a damage over attack speed champ. After that expose weakness is what you want because it'll be more useful in a team fights where you land a very good barrel on their back line so your team does an extra 3 percent damage. Then you choose vampirism over natural talent for an extra little bit of sustain, ending with oppressor over bounty hunter because your barrel and ult makes people slow and makes this mastery effective.

Hi my summoner name is IndianGodKong, i play on the Oceania server, I've been playing and learning a lot about GP, and I've found that this one build that I've been using is going very well for me. I'd like to show and explain this build so You can learn and climb your ELO with one of my favorite champions, Gangplank!

For the runes you want to get 9 attack damage marks, this will boost GP's early game damage as he is a weak early game pick if not played well enough, this is also useful for early game out-trading as you will have an edge on the enemy. With the attack damage marks you also want to get 3 attack damage quintessences for the extra increase in damage and boost of GP's early game as said. You then get 9 scaling health seals for extra health in lane and as GP is a late game champion this helps with his lack of sustain. Finally to top it off you grab 9 scaling cooldown reduction glyphs for late early game cooldown reduction and obviously late game cooldown reduction as they are scaling glyphs. These runes are my runes of best fit comment if you have any suggestions :).

This is my recent match history to show the effectiveness of the build.
As you can see the results are reasonably good, here's why!