What's a "gish gallop" and is it something that should be on standard lists of fallacies?

The Gish Gallop is not a fallacy. Instead it is a technique of overwhelming your opponent with questions or statements, each requiring careful or lengthy rebuttal, more than can be handled in the time or the mental capacity available. It is especially effective against novice debaters.

Oh, and it has absolutely nothing to do with horses.

A "gish gallop" is a method of riding a horse whereby the rider is situated underneath the horse rather than on top of it in the usual way. This method protects the rider from the rain, and lowers the centre of balance of the human-horse system which makes the horse faster, or something.