Gumbald is a gum person who was created by Princess Bubblegum to be her and Neddy's Uncle. Princess Bubblegum mentions him in "Susan Strong," claiming that he had cut down an entire field of taffy trees in a single night for some unknown reason. Finn and Jake were supposed to pull out all the tree stumps that he left behind but only pulled out one before encountering a metallic hatch that led them to Susan Strong.
Additionally, in the episode "Hot Diggity Doom," it is revealed that Gumbald built the log cabin that Princess Bubblegum and Peppermint Butler had to reside in after the King of Ooo took leadership of the Candy Kingdom.
He is first physically seen in "The Thin Yellow Line" being thrown in prison by Princess Bubblegum. The episode does not reveal who he is.
His first present appearance is in Three Buckets, where he collects Fern's remains and asks him if he'd like an Uncle. He is not truly named until the episode "Bonnibel Bubblegum." (in The Wild Hunt he is simply called Uncle G.) It is shown that he is possibly as skilled a scientist as Princess Bubblegum in "The Wild Hunt," as he created the Grumbo, a large, hot fudge shooting Banana Sloth that had been attacking the Banana Guards.
Not much is known about Gumbald in the beginning of the series, as Bubblegum scarcely talks about him. In "Susan Strong," it is mentioned that he cut down an entire field of plum trees in a single night for the Candy Kingdom.
He is mentioned again in "Hot Diggity Doom" when Princess Bubblegum tells Peppermint Butler that Gumbald had built the cabin they were repairing during Bubblegum's exile.
In the "Thin Yellow Line," Gumbald is seen during the Banana Guard's story about the Princess, detailing how dangerous and mysterious the guards see her. In the story, Gumbald is seen thrown in jail, cowering. It is not revealed he is Gumbald until later on.
In "Three Buckets," Gumbald makes his first appearance in the present, as well as his first speaking role. He wears a cape with a large "G" logo on it. After Finn accidentally shreds Fern with his prosthetic arm, Gumbald is seen putting Fern's remains in a bucket, and he asks the pile of grass if he needed an Uncle. He is not named in this episode, but his first line hints at his true identity.
In "The Wild Hunt," he is not seen until the end of the episode, where it is revealed he created the Grumbo antagonist for the episode.
In "Bonnibel Bubblegum," it is revealed that he was created by Princess Bubblegum along with Molly and Chicle, after some time he turns on her and attempts to overthrow her before Princess Bubblegum accidentally turns him into the Punch Bowl.