Gath is a shortened version of gathering which is a small party of people. Usually between 10-35 people, and hopefully with alcohol or weed flowing freely.
James: Hey Rach want to goto Smith's gath this friday?
a metal head, ridiculous metal head, feat. puffy hair
steven "maaaaaaaaaan, that's only gonna burn the paper! let me take it home!"
Mike "steve, you're a crazy gath."