After blocking projectiles enough times, there is a chance for Greg to hatch and spawn a collectible from the Baby Shop item pool, and removing Greg The Egg from Isaac's held items.
Notes
When Greg is cracked open for the first time, it will unlock Devilled Egg and immediately spawn it in place of an item from the Baby Shop pool, if Devilled Egg hasn't been unlocked already. This can happen only once.
The chance for a projectile to crack open Greg starts at 2% after blocking the first 5 hits and is multiplied for every additional hit afterwards.
Only enemy projectiles will be blocked by Greg and cause him to hatch — lasers will not be blocked, nor will they cause him to crack.
The random pickup spawned by Greg The Egg is weighted on the type. The chances for a certain pickup type are as follows:
Box of Friends: Spawns a copy of Greg The Egg. All copies will disappear once one of the eggs crack, however.
Spin to Win: Causes Greg The Egg to deal 6 damage per tick to enemies on contact while Spin to Win is activated.
Abyss: Absorbing Greg The Egg spawns a Abyss Locust that looks like Greg and moves at 33% of normal speed and blocks projectiles. Does not deal increased damage.
Greg The Egg was originally from a renowned familiar mod for The Binding of Isaac: Afterbirth+, Greg The Egg, before gaining a cameo appearance in The Reheated Update of Fiend Folio.
Greg The Egg's first hit to fame started after being shown in a youtube video made by content creator Hutts, THE WORST ISAAC MODS 8. Greg later became popular by fans of Hutts and was synonymous with his The Binding of Isaac content.
The creator of Greg The Egg is also the same creator of Familiar Fly.
Greg The Egg's collectible sprite and flavor text is a reference to Kinder Surprise Eggs.
When Greg The Egg cracks open and spawns a collectible, the item pedestal will have a unique yellow appearance that resembles an egg yolk. Greg will also unleash white creep that represents the egg white.
The white creep is only cosmetic, and will not affect anybody that stands on it.
Devilled Egg and immediately spawn it in place of an item from the Baby Shop pool, if Devilled Egg hasn't been unlocked already. This can happen only once.
Coin
Key
Bomb
Battery
Pill
Card
BFFS!: No effect on Greg The Egg.
Box of Friends: Spawns a copy of Greg The Egg. All copies will disappear once one of the eggs crack, however.
Spin to Win: Causes Greg The Egg to deal 6 damage per tick to enemies on contact while Spin to Win is activated.
Abyss: Absorbing Greg The Egg spawns a Abyss Locust that looks like Greg and moves at 33% of normal speed and blocks projectiles. Does not deal increased damage.
Spindown Dice: Using Spindown Dice on Greg the Egg will reroll it into 
Wrong Warp.
Devilled Egg — Kill
Greg The Egg.
Familiar Fly.
