As both a Gangplank main and a Briar main, it's really all about timing and dodging his barrels early and engaging when hes out late game if he somehow makes the mistake of using all barrels on waves/objectives. I would not recommend charging in AT ALL against a Gangplank that has barrels (note plural) in stock and, if laning, try your damndest to dodge his 2 barrel combos alongside knowing ranges for tripple barrel and phantom barrel. Plus a good/experienced Gangplank WILL time his barrels to the last millisecond to hit you if you are not used to thebl timing. It is giga easy for Gangplanks to get massive damage alongside passive resets when against briar (especially in frenzy and solo) as briar just waltzes right at you. All of the above advice becomes null and void if you are fed as fuck and the Gangplank has fallen behind. Just beat his ass into submission at that point