User blog comment:Kaijusaurus-Rex/List of Season 1 Callbacks in Season 5 So Far/@comment-31194372-20170227211127

Off the top of my head:

-Fiery-eyed Nicole in "The Authority" may be a callback to "The Responsible."

-The entirety of "The Finale" and a scene from "The Curse" in "The Name."

-Carrie repossesses Gumball a la "The Ghost" in "The Mirror."

-Dr. Butt also appears in "The Comic."

-The "Anatomy of a Liar" poster from "The Sock" appears in the hospital room in "The Traitor."

-Gumball plays "Butt Puppets" in "The Test".

-Darwin kissing Rachel and Gumball in the montage from "The Matchmaker."

(There's a lot more, but I can't think of them at the moment. This is good enough for now.)