Good question. Named after Duane Gish, the Gish Gallop is a special case of fast talking (the technique famously employed by Snake Oil Salesman (and contemporarily with used car salesman) that confuses people with fast long strings of words long enough to convince them to buy snake oil). In the classic Gallop, a long string of assertions are thrown out in an argument, most of which have questionable sources if any at all; consequently addressing all of the issues raised with the depth that they deserve is practically impossible: it would simply take too long.
In spoken debate Gish would reel out so many points in a short time, his opponent would be in the impossible position of either trying to quickly refute all the points (and failing because he hasn't enough time) or letting some points slide (which might convince people that the some arguments were left undiscussed because they are irrefutable).
It works best as a cheap rhetorical trick, but it can translate approximately into the written word.
What are the signs someone should look for to be aware of the Gish Gallop?
If there are a lot of statements being made, or if the topic seems to swing from pillar to post, then you should be aware that a Gallop is in place. If the person is talking very quickly in a way that is well rehearsed, making claim after claim after claim then you have yourself a Gish Gallop.
Every point could be right, of course, but if you are really interested in things - explore both sides of the issue at stake, look up sources and try reasoning it out with a critical mind. The problem is that most folk don't have the time to do that so the best defense is awareness. In written form it is better to find articles that fully discuss both sides of any controversial issue and tries to explain why their explanation is the best one.
In spoken form, the only real defense is to realize what just happened and show that several things the person said are grossly inaccurate (if that is the case, of course) and question on that basis the credibility of the other assertions.