Instructions
Reach level 25 on Zombie Farm. You can not make gray zombies until you reach this level.
Purchase a zombie pot from the market. The pots are available for either gold or brains.
Tap the zombie pot and confirm that you want to combine zombies.
Tap two zombies that are roaming on your farm to place in to the zombie pot. Tap "Yes" to confirm you want to combine the two zombies. The zombies must be the same type to create a gray zombie.
Wait for the zombie pot to notify you that the zombie is ready. Tap the pot to release the zombie.