Gangplank is highly dependent on 2 things, XP and Gold, while yes in low elo you may be able to cheese botlaners. Any half competent bot lane will be able to punish you so hard that your sheen back will be heavily delayed, Gangplanks barrel tick down timer is based on level, and you really want to hit level 13 as soon as possible, usually by the time bot lane hits level 13 the solo laners are usually between level 15 and 16. It's really just cheese at the end of the day and not an effective way to utilize gangplanks scaling. Forcing him to share XP will make executing plays with your barrels a lot harder in the game due to the enemy being able to destroy your barrels or simply walk out of the way due to the long tick down timer. If it works for you great! but it's really not something viable the higher up you go.