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. Sense Magic. The gilman senses magic within 120ft of it at will. This trait otherwise works like the detect magic spell but isn't itself magical.
Multiattack. The gilman makes two pincer attacks. If the gilman is grappling a creature, the gilman can also use its tentacles once. Pincer. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 11 (2d6+4) 2d6+4 bludgeoning damage. The target is grappled (escape DC 14) if it is a large or smaller creature and the gilman doesn't have two other creatures grappled. Tentacles. One creature grappled by the gilman must succeed on a DC 13 Constitution saving throw or be poisoned for 1 minute. Until this poison ends, the target is paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.