Spawns a demon familiar that's attached to Isaac, moving in the direction Isaac shoots, and automatically shooting (whenever Isaac is shooting) in the direction of the closest enemy to it, using Isaac's damage, tears, range, shot speed, and specialtear effects.
The item is initially held over Isaac's head until he fires, which launches the familiar, dealing 3x Isaac's damage.
If Isaac has any tear modifier that requires him to charge his shots (like Brimstone), Gello will automatically "release" the fire button for himself every time he is fully charged. If Isaac releases the charge button himself, Gello will do that too, regardless of his current charge of an attack.
Gello's tears deal 0.75x Isaac's damage, or 1.0x if used by Lilith or Tainted Lilith.
Notes
Tainted Lilith attacks in a way nearly identical to this item, except she can recall it and launch it again.
If used multiple times in the same room, the Gellos will eventually overlap perfectly and appear as one after some movement, but all remain functional. This makes Gello exceptionally powerful in Greed Mode, Boss Rush, and against The Beast.
Synergies
Book of Virtues: Spawns a red wisp on the middle ring. The wisps will orbit Gello instead of Isaac while it's active.
Tainted Azazel: Each time Gello starts charging an attack, he will "sneeze" just like Tainted Azazel does, cursing enemies in front of him. This allows Tainted Azazel to keep enemies cursed without needing to come up to a dangerously close distance to them.
Tainted Lazarus: The familiar persists when switching states.
Uranus: When the baby is launched, any enemies it kills in its trajectory will freeze and any frozen enemies the baby is launched into slide if Isaac is moving, similarly to when kicked by Isaac.
Using ? Card on Gello will still result in the charges being used once Gello is whipped out. The ? Card will be used up as well, despite providing no free use of the item.
Brimstone), Gello will automatically "release" the fire button for himself every time he is fully charged. If Isaac releases the charge button himself, Gello will do that too, regardless of his current charge of an attack.
Lilith or 
Tainted Lilith.
Boss Rush, and against 
The Beast.
Book of Virtues: Spawns a red wisp on the middle ring. The wisps will orbit Gello instead of Isaac while it's active.
Tainted Azazel: Each time Gello starts charging an attack, he will "sneeze" just like Tainted Azazel does, cursing enemies in front of him. This allows Tainted Azazel to keep enemies cursed without needing to come up to a dangerously close distance to them.
Tainted Lazarus: The familiar persists when switching states.
Uranus: When the baby is launched, any enemies it kills in its trajectory will freeze and any frozen enemies the baby is launched into slide if Isaac is moving, similarly to when kicked by Isaac.
Car Battery: No effect.
Incubus.
Bug!
? Card on Gello will still result in the charges being used once Gello is whipped out. The ? Card will be used up as well, despite providing no free use of the item.
Tainted Lilith Unlocks 
Delirium Unlocks 
The Binding of Isaac: Rebirth
