Shortening of the word Gathering.
Its like a party but smaller, usually with alcohol, and on the occasion drugs.

A casual invitation / threat used by an individual that is not afraid of fucking someone up.

Young peopel use the word Gatho instead of party to sway parents to let them go. It is meant to convey a smaller and more innocent night with friends, but in reality is just another word for party. They also dont realise how douchey they sound when they say gatho, and also how much it makes me want to punch them right in the face.

A Gatho is short for "a gathering" of friends for the purpose of having a good time. Usually for the consumption of alcohol.