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.
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.
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 Lilith: This may just be a visual effect, as with Tainted Lilith also clearly appears to "sneeze" when Gello comes out to start an attack yet does not curse enemies.
Gello is a revenant in Greek mythology who causes infertility, miscarriages, and infant mortality.