Once it appears, it goes on a rampage. It remains enraged until it demolishes everything around it.
Johto Pokedex entry - Pokemon Silver

**Wild GYARADOS appeared!
...

Well, you're fucked.**

As rare a sighting as a wild Gyarados was, it never failed that a few trainers would always mysteriously disappear during the Gyarados annual breeding and egg laying seasons. Eventually the people of Mahogany Town thought it best to cut off access to the Lake of Rage for the sake of the townspeople as well as the trainers.

Big.. violent.. wild... and you're supposed to catch it. Riiiiiight.

Please also try to remember that "You're Royally Fucked" also comes in Legendary flavors.

Thank you, and have a nice day.