A gate burst happens when a Mythos card is drawn during the Mythos Phase and the card's Gate Symbol is highlighted in orange. This immediately destroys any active Elder Sign sealing the location, so that a gate and a monster (or two if it's a five or more player game) appear there as if there were no Elder Sign token present.
When a gate burst is played, all flying monsters are moved regardless of dimensional symbol.
Gate Bursts may also activate special abilities on items and bosses. ((Image needed))
Gate burst do not appear in the base Arkham Horror game, only in expansions.