Sharp Hearing. The Gremlin has advantage on Wisdom (Perception) checks when hearing steps.
Group Rampage. The Gremlin rolls [roll]1d4[/roll] as attack bonus if at least two of the Gremlin's allies are within 5 feet of the creature and the ally isn't incapacitated.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage. Must succeed on a DC 10 Constitution saving throw or contract a disease. Until the disease is cured, the target can’t regain hit points except by magical means, and the target’s hit point maximum decreases by 3 (1d6) every 24 hours. If the target’s hit point maximum drops to 0 as a result of this disease, the target dies.
Fog Eater. While in the 'Fog', can recover [roll]1d4[/roll] Hit Points with one complete action.