Slang abbreviation of 合コン, a party for students from different colleges or groups to meet. Rarely written as ＧＫ in this sense; more often spoken or typed in kana.
A senior invited me, so I went to a mixer with students from another university.
Etymology
Abbreviation derived from the initial letters of ゴールキーパー (goalkeeper), ゴールキック (goal kick), and 合コン (joint party). The reading ジー・ケー reflects the English letter names G and K.
