A gish gallop is lying so fast that by the time whoever you're arguing with manages to refute all your lies the conversation has already been over for a while

It's a modern term (named after creationist Duane Gish) for an old-as-dirt oral debate tactic: spew as much bullshit, lies, half-truths and occasional facts as you can in order to bamboozle your audience, as well as put your debate opponent into a position of needing to debunk your nonsense rather than argue their own points.