This is an idea for season 8. It contains 4 episodes.

The Alien:

Gumball thinks that Darwin is an alien.

The Skunk:

Gumball finds a skunk which causes him to smell.

The Truth:

Gumball admits to Penny, that he is in love with her.

The Graduation:

Series finale:

Gumball gets to graduate on the last day of school, but Darwin didn't graduate, causing Gumball to go to College without him. In the end, Gumball leaves and writes a note to Darwin, saying: "see you soon!". And everybody said bye to Gumball.