I disagree. Engaging implicitly validates the argument. If you refute X, they can just come back with "yes but what about Y?" and, should you eventually get bored or run out of time, they use your failure to properly rebut as evidence of their thesis.

I'm also not familiar with any argument that a Gish gallop must make claims not supporting a single thesis, or that they must be hard to refute (as opposed to simply harder to refute than to state). Whatever it is, I don't subscribe to it, and I don't believe those are the traits which make the argument style inappropriate.