I think the Gish Gallop is one of those situations where it's entirely valid to deploy an ad hominem - and the more ruthlessly, the better - since the only viable and time-efficient approach is to discredit the person so that everything they say is ignored by as many people as possible[0].
Some people are - understandably - squeamish about ad hominems, but if a person is a known bullshitter, liar, or simply parrots whatever they've heard elsewhere without regard for accuracy, it's often the best way of dealing with them becauase you won't have time to deal with the merits, or lack thereof, of their arguments individually.
The problem comes when the ad hominem is your default approach as, again, it so often is in the media and politics... and, indeed, in work and everyday life. I don't enjoy seeing it deployed against people acting or arguing in good faith, even where I disagree with them.
But if somebody is acting or arguing in bad faith, have at them.
[0] Yes, there might be some truth mixed in with the lies and bullshit, but you'll be able to get that same truth from better and more reliable sources. It's a better use of time and mental energy to simply ignore the unreliable source.