This is similar to the Free gumball on Shadows of Evil. All you need to do is make your way to the teleporter room in the Undercroft. To the right on the shelf we will have a pot with an undeveloped plant. You want to toggle that plant by pressing square or X. Now it will be in your inventory. What you need to do now is teleport back in time. There are two ways you can do this. Either through the clock tower by putting it to the time 9:35 or the Main Easter Egg steps which involve shooting the Wisps of the teleporter than shoot all four purple orbs. When you teleport back in time place the plant where you found it. Now since you placed the ungrown plant in the past. When you teleport to the future it will grow with the gumball in it's leafs. Just toggle it when you seen it fully grown.