My Carbuncle loses agro all the time but never seems to use Gouge instead.
You appear to have your two Carbuncles mixed up. Gust is the "auto-attack" spell of the Emerald Carbuncle, summoned with the spell Summon. Gouge is the "auto-attack" spell of the Topaz Carbuncle, summoned with the spell Summon II. They function differently, and serve different roles - Emerald Carbuncle is best suited for group activities (like dungeons, FATEs, etc.), whereas Topaz Carbuncle is best suited for soloing (since all of his abilites have increased enmity generation).
Note that Summon II and the Topaz Carbuncle aren't earned until level 15 upon completion of the quest Topaz Teachings, available in the Arcanist's Guild in Limsa Lominsa.