It takes the form of a swarm of bugs that will chase and attach itself to the player character's head. While attached, it will decrease the player's Sanity and deal 1 damage every 10 seconds, starting 30 seconds after being attached. It will follow the player until it finds a non-fire light source to attach to, either at Dusk or Night (such as Thunderbirds, Town Houses, or Glowflies).
The swarm isn't a status effect, but is an actual entity that is flying around the player. When force attacking, the swarm can only be hit if there are no other targets around. Attacking them directly won't deal any damage, possibly because they are not a single entity but a swarm and therefore can dodge attacks quickly. However, they can be killed by other means:
- Spraying a cloud of Bug B'Gone,
- Freezing them with an Ice Staff or Chilled Amulet, and then attacking them normally (this action does not decrease durability of the Staff but still drains sanity as usual),
- Leading them into the Gas Rainforest biome
If a Gnat Mound is destroyed, the swarm which comes from it will spawn a new mound after some time, similar to Moleworms.
The swarm still attaches to the player even when your character dies and resurrects with the power of a Meat Effigy, Touch Stone or a Life Giving Amulet. It's possible to carry a Gnat Swarm to connected worlds by using the Skyworthy
- Overcharged WX-78 players should be cautious, as they shine brighter than the other light sources, and therefore the Gnat Swarm will never leave them on its own.
- When playing as Wendy, Abigail will instantly attack Gnat Swarms and won't stop doing so, unless the player goes into a cave in which Abigail is going to follow the player. After that, she follows the player as normal. Abigail is not able to kill the Gnat Swarm, however. At night, the swarm may attach itself to Abigail as she is a bright, non-fire light source.
- Strangely, using a Telelocator Staff and a stocked Telelocator Focus on a attaching swarm result in completely destroying it (whether a mob or a player being attached), while using on a wandering swarm result in teleport it to the Telelocator Focus. Making this an expensive way to kill Gnat Swarm.
- Iron Ore can be renewable outside of a Hamlet world by carrying a Gnat Swarm then using an Ice Staff to freeze it when a desired location is reached.
- Maxwell does not lose any health from Gnat Swarms, and the sanity loss they normally cause is negated by his dapperness.