Gluttony is a new skill Rimuru acquired after absorbing the orc disaster. Using the unique skill Degenerate (received from Shizu) Great Sage combined Starved and Predator into the unique skill Gluttony which now has the effects of both. In this specific case gluttony was more of a command word given to Great Sage to essentially launch an auto-fight program (similar to how Great Sage fought the orc disaster for him). The first time we actually see him use Gluttony is during the fight with Charybdis, it's one of those things that is explained in the light novel but is glossed over in the show due to time constraints.