The Gish Gallop is a colloquial term used to describe a practice employed by some during debates. It was coined in reference to Dr Duane T. Gish who was an American creationist notorious for overloading his opponent with so many points to refute that the other person finds it difficult to refute them all in her allotted time. It is essentially an underhand debating tactic used to unsettle and confound the opponent and make it seem he has so many points in favour of his proposition that it must be true.
This term is usually given to those debating against established science. If any point is left unrefuted, then the Gish Galloper claims victory in the debate. As a tactic it is not restricted to science though, it is also used by unscrupulous politicians in public debates.
People using this tactic can be exhausting to debate online and will keep churning out walls of text for the other person to knock down. It can be an effective deterrent for engagement with someone.