There are untold ways of arguing in bad faith on the Internet, and this is one of them. The Gish Gallop is flooding an argument with weak evidence in such a large quantity that responding to it all is burdensome.
There are a few different ways that this can manifest. In both spoken and written debate, the Gallop can take the form of a large number of points that are interleaved with, if not entirely made of, bullshit. Responding to all of them in a timed debate is impossible, and responding to all of them even given unlimited time is exhausting. Not responding to all of them gives the perpetrator of the Gish Gallop ammunition to accuse you of cherry picking arguments. A sufficiently advanced Gallop will attack the argument with such a wide variety of points that it is sure to extend outside your domain of expertise and force you to argue against it from a position of weakness.
In addition, there is a form of the Gish Gallop enabled by the Internet, where the Galloper links to an existing, lengthy text or video as the proof of his side, and is deliberately vague about which portion of this linked content is relevant. Again, refutation of the entirety is beyond what could be reasonably asked of a person, partial refutation is an invitation to accusal of cherry picking, and a request for clarification is met with the response of “It’s not my job to do your research for you.”
The effectiveness of a Gish Gallop stems from the fact that making an assertion involves much less cognitive load than either proving or disproving said assertion. The Galloper is counting on the person attempting to refute it not having the time or energy to respond to every point presented, because it is an order of magnitude greater than the original set of points. In addition, on the Internet the same collection of Gallop points can be recycled endlessly; there is no first reading required to make the points, while responding to them does require reading through all of them.
If you sense that you are being drawn into one, best to walk away, call it out for what it is, and/or head the argument off with an ad hominem attack and a generalization of all the points made as bullshit (because they likely are). Also, don’t argue on the Internet, you fools.
Duane Gish received his Ph.D. in biochemistry from UC Berkeley in 1953, and after his postdoctorate reserach there, found positions at Cornell University Medical College, the Upjohn company, and San Diego Christian College. Gish was also a devout creationist, founding the Creation Research Society in 1963 and later serving as the vice president for the Institute for Creation Research until his retirement from the position in 2005. Gish’s 14 peer reviewed journal papers were within his field of biochemistry and not on evolutionary biology, but his later oeuvre of 15 books could be summed up as creation FUD against evolutionary theory.
Eugenie Scott of the National Center for Science Education coined the term “Gish Gallop” after numerous contacts with Gish, starting when her mentor, James Gavan, debated Gish at the University of Missouri in 1974. Dr. Scott presented the term in a 2004 article about public engagements with creationists, which you can read here.