I recommend this guide for you if you play with Soraka or Sona in top lane, either you can go solo top and play defensive (inc. buy wards, mana pots -no push! only last hits)
Beware of ganks when you solo top and if your friend is going to engage in mid 1v1, you activate your ulti so he can reach his kill and u get assist!

Gangplanks farming is very important! You need many creep kills if you want your gear as fast as possible ofc. Allways get last hits, when you are lvl 7 go get blue buff if its okay with ur jungle friend.

Teamfights wouldnt be so good unless you got your ulti but dont engage first. Gangplank should play careful in teamfights because if he is the targed he will die in a stun.
So stay until at least 2-3 in your team engage and start teamfight then u shoot parrley and attack with basic/auto attack

When Gangplank [play vs (ex. Veigar, Annie, Rammus, Brand, Tryndamere or Malzahar) -he should play very careful because these are heavy crowd controls (cc). The 'w' spell is very important because it remove all cc's so allways save your fruit when there is like 3v3 mid and they have cc!