Imagine your position as a fortress, and solid counter-arguments are cannons. A "Gish Gallop" is like bringing automatic tennis ball launcher a quarter mile from the walls of the fortress and turning it on. The defender comes out and whacks the tennis balls away. No chunks are torn from the walls of the fortress. But to anyone watching, it sure looks like the attacker is wearing out the defender; with so many points, some must be true? In politics today, the Gish Gallop is used most often on climate change. The reason that almost all the world's scientists are wildly wrong changes from moment to moment and post to post, but there are always many reasons. It is a challenging tactic to counter. I think the best techniques should aim to slow the conversation down. In the analogy, keep your eye on the first ball. Don't hit the ball away, see where it lands: ask them follow-up questions, get them to keep expounding.
If you have examples of successfully dealing with gish gallop attacks, either one-on-one or in front of an audience, please share them.