Duane Gish is an American biochemist and a prominent member of the creationist movement. Gish has been characterized as using a rapid-fire approach during a debate, presenting arguments and changing topics very quickly.
Eugenie Scott, executive director of the (US) National Center for Science Education, has dubbed this approach the "Gish Gallop," describing it as "where the creationist is allowed to run on for 45 minutes or an hour, spewing forth torrents of error that the evolutionist hasn't a prayer of refuting in the format of a debate". This is the context in which the term is most often heard.
She also criticized Gish for failing to answer objections that his opponents did actually manage to raise.
The technique is far from rare, and far from acceptable. The solution to the problem is very clear in the formal debates. It is obvious that it is the job of the chair of the debate to give both parties an equal opportunity to make their argument and it appears to me that the gish gallop is a sign of failure to keep control of the debate.
Small note: Have you ever noticed this being used in the comments on blogs? :))