The Existence Of The Kaijin Part 1 is the 1st epsiode of The Amazing-Word of Gumball: The Kaijin
Gumball, Darwin and Anais were in the living room just relaxing, until Anais found a note sent by an unknown writer about human-sized beings refered to as "kaijin", then asks what are Kaijin. Gumball answers who cares. But then Anais reads the note about them, then Gumball gets a shock, then told Duncan everything but Duncan didn't understand what they saying, then they just gave up, but Gumball then decided to tell penny instead. So then, the four went to find Penny and tell her the news. Once they did that they went to the forest.
So then the four went to investigate who or what are Kaijin, until they find the first Kaijin monster, a gazelle-like creature called Gigazelle. Then the four the four ran for lives and then separated from each other. Once they got separated, Gumball finds the Kaijin leader, an alien being named Akudos-Gill, then after a few seconds he disappears.
While Darwin was running, he bumps into Gigazelle, then the episode ends.
Gumball: Would you guys even try this heap of junk?