Gaper

Gapers are monsters that resemble Isaac but have blood instead of tears, and their skin is paler. Gapers are commonly found in The Basement and Cellar.

Gapers will walk towards Isaac by the most direct route possible. If there is no way to reach Isaac, due to obstacles such as rocks or poop blocking the way, they will stand still until a proper route is created.

Sometimes they turn into Pacers or Gushers after dying.

Variations

Frowning Gaper

Frowning Gapers look identical to regular Gapers but have empty eye sockets that appear closed before they get close to Isaac. When awake, they have the same head sprite as Horfs. Frowning Gapers move quicker than Gapers, and the chance for them to turn into Pacers or Gushers is higher.

Flaming Gaper

Flaming Gapers behave identically to regular Gapers, though they move at a faster pace, but not as fast as Frowning Gapers. They appear to be on fire, but despite this they aren't negatively affected by it in any way. They mainly replace most of the regular Gapers in the Burning Basement.

Cyclopia

The Cyclopia moves slower than normal Gapers but leaves behind a short-lasting trail of Red Creep and occasionally fires a blood shot towards Isaac.

Gurgle

Gurgles are considerably tougher enemies that look identical to Frowning Gapers aside from their green coloration, and are very common in Catacombs. When they get near Isaac, they fire an Ipecac shot at him. Killing it has a small chance for it to become a Splasher. These shots will kill other enemies in their Area of Effect, but Gurgles themselves are immune to their own shots.

Like other enemies' explosive projectiles, Gurgles' shots can be used to shatter Rocks or open Secret Rooms by making them fire at the appropriate area.

Gurgles are somewhat more predictable and accurate in their shots compared to Guts and Walking Guts, because they will always shoot at Isaac's position. However, the timing of their shots is unpredictable. As such, an effective strategy of delivering explosives to a particular area (e.g. tinted rock or potential Secret Room entrance) is to stand still in that position as long as possible before moving to avoid getting touched, to prompt the Gurgles to shoot their projectiles at you.

In Repentance, Gurgles have a hunched posture, like how they did in the original game.

Crackle

Crackles are a variant of Gurgles found only in Ashpit. They walk towards Isaac similar to a Gurgle, but instead of shooting an explosive shot, they shoot a ball of fire that releases 4 lines of fire in all cardinal directions when it touches the ground.

Nulls

Nulls appear fairly rarely in Chapter 3 and below. They behave similarly to Frowning Gapers, but walk faster and have the ability to phase in and out of visibility.

Notes

In Chapter 3, Nulls are most commonly encountered in a 2x2 room layout, where six of them are encased in the center, unable to chase Isaac. Isaac has to blast away some skulls first to open the path to them. Flight, as well as spectral tears, do very well here, as the player can fly in, drop a bomb, and leave, and the enemies will remain stationary where they were last chasing the player.
White Champion Nulls may not be able to turn invisible while invincible.
They are referred to as "Nulls" in the bestiary which goes both for its singular and plural usage.

Blue Gaper

Blue Gapers behave similarly to regular Gapers and are mainly spawned during the Hush boss fight. They can rarely appear in Bedrooms and I AM ERROR rooms.

They can rarely appear in a closet in Basement, Depths, and Necropolis. Blue gapers can also be found in the unique rooms created by Blue Key and in ??? Red Key rooms.

Greed Gaper

Greed Gapers behave similarly to regular Gapers and are spawned during the Ultra Greed boss fight.

2 Greed Gapers may rarely replace the shopkeeper in Shops.

2-4 Greed Gapers have a chance to appear in Treasure Rooms.

Greed Gapers no longer appear in treasure rooms and are replaced by the Bulb Fly. They can, however, still appear in certain Super Secret Room layouts. Greed Gapers can also appear in Ultra Secret Rooms.

Deep Gaper

Deep Gapers are a new Gaper variant that appear in Downpour and Dross. They will emerge from the floor once all previous enemies on certain room have been killed. Deep Gapers behave like normal Gapers after emerging.

This enemy has 14 design variants, 7 for the Downpour floor and 7 for the Dross floor. These variants are purely cosmetic with the exception of one that has a Small Leech sticking on its head, which will detach from it and chase Isaac when the Gaper is killed.

Blurb

Blurbs appear in Downpour and Dross, with a different appearance for either environment. They initially walk slowly towards Isaac, but after taking some damage or getting close to him, will release the water stored in its mouth as a chaotic burst of tears. After firing their tears, they act like regular Gapers and chase Isaac faster than before, attempting to deal contact damage. They will still spit out their water if killed in an instant with high damage, firing towards Isaac even if he is not being targeted.

Wraith

Wraiths appear in Downpour and Dross with a different appearance for either environment. They behave like regular Gapers but are detectable only by their reflections and the ripples they make in the water. The occasional lightning strikes in Downpour will also make them briefly visible.

They are visible in the Mirrored World, but their reflections will be invisible instead, appearing when lightning strikes.

Rotten Gaper

Rotten Gapers are a variant of Gapers found only in Corpse. They chase Isaac faster than regular Gapers, and have slightly different behavior depending on their facial expression.

If its eyes and mouth are normal, it chases Isaac slowly.
If it has a flattened eye, it chases Isaac slightly faster than a Globin.
If it has an empty eye socket, it chases Isaac quickly, slightly slower than a Gazing Globin.
If it has its eyes closed, it chases Isaac slowly, spawning Small Maggots once every two seconds.
If it has its tongue sticking out, it chases Isaac similarly to Lust, gaining speed as it moves but changes directions slowly, losing momentum as it does.

Twitchy

Twitchies are a Gaper/Trite hybrid found only in Corpse. They alternate between chasing Isaac and rapidly leaping at him. They leap at Isaac 1-3 times before returning to chasing him on foot, and their leaps can target him from almost across an entire normal-sized room. Their heads also occasionally twitch violently (hence the name), although this is purely cosmetic.

Level 2 Gaper

Level 2 Gapers appear in the Corpse. They look like a large, decaying version of Frowning Gapers.

They behave much like Frowning Gapers, however once they take enough damage, they become split into a Level 2 Horf, which spits Small Maggots at Isaac, and a Level 2 Gusher, which runs around the room and spawns Small Maggots.

Gallery

Frowning Gaper 1.png

A Frowning Gaper before It moves close to Isaac.
Null 1.png

A Nulls before it moves close to Isaac.

Trivia

When the Head of the Keeper is picked up as Keeper, he will look similar to the Greed Gaper.
The appearance of Cyclopias is reminiscent of the fatal birth defect of the same name, which is characterized by the failure of the embryonic prosencephalon to properly divide the orbit of the eyes into two cavities.
A Stoney makes the same sound as a Large Champion Gaper.
Frowning Gapers and Nulls don't make noises until they open their eyes and start chasing the player quicker.
Greed Gapers are also known as Greedlings, a name given by a card of it from The Binding of Isaac: Four Souls, although it's seldom referred to as a "Greedling" by players.
Gapers and Flies are some of the monsters with the most variations. The Gaper has 14 variations.
Blurbs spawned outside of Downpour will be skin-toned and Wraiths will appear translucent.
Rotten Gapers are the only Repentance enemy to lack a Bestiary entry, despite being naturally spawnable and not a monster variation summoned by a boss.
Twitchys in Antibirth have a more melty and greenish appearance than in Repentance.

Bugs

Bug!	If a Dark-Red champion Frowning Gaper does not become a gusher or pacer and reforms, the head won't appear until the Gaper sees the player. The same goes for a normal Gaper, however, the head won't appear once reformed.

Bug!	If an Explosivo tear is stuck to a Null when it becomes invisible, the tear will fall to the ground.

Bug!	Rotten Gapers don't appear in the Bestiary in any form.

Bug!	If a Champion Gaper is killed and it turns into a Pacer/Gusher, it will no longer be a champion.

The Binding of Isaac: Rebirth