Same word as non-gregarious:

backward, bashful, coy, demure, diffident, humble, meek, modest, nongregarious, not forward, quiet, rabbity, recessive, reclusive, reserved, restrained, reticent, self-effacing, shrinking, timid, timorous, unaffable, unassertive, unassuming, unexpansive, unsociable, withdrawing, withdrawn