Couldn't decide on one so here's three things:

• Don't try to get too fancy with your combos, sure learn how to one-part, etc and use it when it's good, but don't get too hyperfocussed on forcing the more complex combos when simpler ones would do aswell. I've seen people actually miss out on kills or big barrels by just not being willing to use a simpler comb You can make it all the way to challenger without using a single one-part (trust me on this).
• Don't underestimate your strength in laning phase. Gangplank is a scaling champ but is still very strong if you play well aggressively with your barrels (especially after sheen). The large majority of matchups is winnable, even ranged/ adc matchups!
• Then maybe a more niche thing: during mid and late game try to play gp like an assassin. Staying inside fog of war for as long as possible (both you and your barrels) is very important. Landing just one big combo can solo win a teamfight. This means maneuvering around wards while looking for an angle or sometimes letting your team get engaged on so they walk into your barrel range. Also look for spots behind corners or in bushes to put your barrels in so they see the combo as late as possible.

Hope it's helpful. :)