The Gish Gallop is the debating technique of drowning the opponent in such a torrent of small arguments that their opponent cannot possibly answer or address each one in real time. More often than not, these myriad arguments are full of half-truths, lies, and straw-man arguments — the only condition is that there be many of them, not that they be particularly compelling on their own. They may be escape hatches or “gotcha” arguments that are specifically designed to be brief, but take a long time to unravel. Thus, galloping is frequently used in timed debates (especially by creationists) to overwhelm one’s opponent.
Examples are commonly found in “list” articles that may claim to show “100 reasons for” something, or “50 reasons against” something. At this sort of level, with dozens upon dozens of minor arguments, each individual point on the list may only be a single sentence or two, and many may be a repeat or vague re-wording of a previous one. This is the intention: although it is trivial amount of effort on the part of the galloper to make a point, particularly if they just need to re-iterate an existing one a different way, a refutation may take much longer and someone addressing will be unable to refute all points in a similarly short order. If even one argument in a Gish Gallop is left standing at the end, or addressed insufficiently, the galloper will attempt to claim victory.
The term was coined by Eugenie Scott of the National Center for Science Education, named after creationist Duane Gish. Creationists are fond of it; see “101 evidences for a young age of the Earth and the universe” for example, which is perhaps the most stunning case. Sam Harris describes the technique as “starting 10 fires in 10 minutes.”