This is a term that originated in debating circles in America to describe a particular debating strategy. The tactic is to bombard your opponent with a succession of arguments based on wishful thinking rather than science, on rumour and hearsay, on fake news. No one can attempt to rebut such a flood of nonsensical arguments so opponents are left speechless as the torrent of misinformation engulfs them.
The tactic takes its name from Duane Tolbert Gish, 1921-2013, who, in defending creationism, produced this style of debating. The term now applies not just to formal debating but to interviews where the interviewee rattles off one untruth or piece of pseudoscience after another, hardly pausing to take breath. The noun has developed a verb form so you can talk about someone Gish galloping in an interview. For the moment we seem to be retaining the capital G in Gish but already the tendency is evident to make it lower case.