Bite

Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 40 (6d10 + 7) piercing damage. If the target is a creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the gulper behemoth can't bite another target.

Swallow

The gulper behemoth makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the creature is also swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the behemoth, and it takes 14 (4d6) acid damage at the start of each of the behemoth's turns. If the gulper behemoth takes 20 damage or more on a single turn from a creature inside it, the behemoth must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the behemoth. If the behemoth dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Sonic Pulse (Recharge 5-6)

The gulper behemoth expels a sonic pulse in a 60-foot cone. Each creature in that area must make a DC 16 Constitution saving throw. On a failure, the creature takes 21 (6d6) thunder damage and is stunned until the end of its next turn. On a success, the creature takes half the damage and isn't stunned.