From 一人(ひとり) (hitori) + 勝(か)ち (kachi, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 勝(か)つ (katsu).”). The kachi changes to gachi as an instance of rendaku (連濁).
ひとりがち • (hitorigachi) ←ひとりがち (fitorigati)?