Great Gideon is a boss that appears in the Mines. He is always fought in a vertical double size room.
Great Gideon cannot be damaged except by killing the waves of enemies he summons, which deals one point of "damage" to Gideon each time a wave is killed. He has a wide range of attacks:
- Spawns various enemies around the room.
- Spews a barrage of flaming shots in a vertical line from Gideon to the lower end of the room, which leaves fires that can damage Isaac wherever they land.
- Fires a rock wave down the room towards Isaac, destroying parts of the floor.
- Fires a spread of 5 shots towards Isaac, along with a large shot that then bursts into eight tears in all directions.
- Sucks Isaac and rocks from the walls towards him.
- Using Chaos Card on Gideon will kill him and open up a crawl space underneath him containing several items and chests.
- If all the enemies in a wave are killed before he channels an attack, instead of attacking he will spawn the next wave. Through this, it is hypothetically possible to fight the Great Gideon without him ever directly attacking Isaac once.
- Great Gideon, like several other bosses introduced in Antibirth, is based off of a group of common monsters encountered on the various floors. In Gideon's case, the boss is designed after the Stone Grimace and its counterparts.
- Like all Grimace monsters, Gideon is practically invulnerable to direct attack, and instead of dying, simply deactivates when Isaac kills every monster he can, as shown by the fact that the boss is beaten by defeating waves of normal enemies rather than by killing Gideon itself.
- The five-shot spread fired directly at Isaac and the bursting large shot mirror the behavior of the regular Stone Grimace.
- The barrage of burning Ipecac-like shots is based on the attack used by Vomit Grimaces.
- The move during which Gideon sucks Isaac towards himself is similar to the behavior of the Gaping Maw.
- Gideon's ability to send a linear rock wave along the room is taken from the Quake Grimace, a new kind of Grimace added in Antibirth.
- The holes in Great Gideon's left eye may be a reference to the Blaster, a new monster added in Antibirth.
- Great Gideon has his unique version of a health meter texture.
- The crawl space that opens up when using Chaos Card on Gideon is titled 'Gideon's Grave' in the code.
- According to NotYourSagittarius, this is a direct reference to Scott Pilgrim, a series of graphic books.