Obtained through the Quest "Legless Spyder" at the end of the quest you will receive the gun.
The Anti-Gadoid Gun is a dud and cannot fire any type of ammunition or do any damage to enemies.
When fired, it alerts special infected which come to your location. To use this to its full potential I recommend using silent weapons when enemies are using loud weapons. **Example** I used this gun to clear out one of Rais Mens AirDrops fired it and then ran the infected came and killed them all then I killed the weaker infected and took the loot!
EDIT: And also, you can use the weapon to stun night creatures, giving you enough time to kill them, and hesitate the children for screaming.