Homebrew
Limited Amphibiousness. The gilman can breathe air and water, but it needs to be submerged at least once every 4 hours to avoid suffocating. Stone Camouflage. The gilman has advantage on Dexterity (Stealth) checks made to hide in rocky terrain.
Multiattack. The gilman makes one attack with its tentacles. If that attack hits, the gilman can make one beak attack against the same target. Tentacles. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (2d4+2) 2d4+2 slashing damage. Beak. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4+2) 1d4+2 piercing damage.