Farming with Gangplank is actually very easy. Just keep some distance from the minions and don't waste your Q poking the enemy, unless you intend to start a 1 vs 1 soon after that. Use your Q to farm whenever possible, mainly when you're with Avarice Blade, since it will stack even more gold for you. Getting one of these for farming even after you have already bought Statikk Shiv, is a very good catch, if your lane is being kinda hard, it will give you gold for around two minions. Sorry for the bad english.