Green’s Dictionary of Slang

gack adj.

[? echoic of a sound of distaste]

unappealing, pretentious.

1982

G.V. Higgins Patriot Game (1985) 186: ‘Gack,’ Riordan said. ‘Ferns, Harvards. Interior decorators. Toy food. Bean sprouts. Tofu. Vietnam food for the intellectually chic.’.