Make sure you’re gambling daggers, and not dirks. Gull is a dagger, so if you are gambling the lookalike “dirk” class, you’ll never get a Gull.
When gambling, the item level you can get is Clv -5 up to CLv +4. So to gamble Gull, an iLV 6 item, the optimal level to do this is 11 since you won’t receive anything lower than iLV 6 in a gamble and the lowest iLV possible for gambling is also the most common.