Gumbo is a chimp-like imaginary friend dressed like a private investigator. Bloo encounters him in "Cuckoo for Coco Cards".
Despite his deep voice and serious appearance, Gumbo sports many odd characteristics. He's fixated on his own height and likes mentioning it, and often asks others for their height for "future reference". His feet are encased in cinderblocks for reasons unknown, but it's likely so he can stay balanced (or, since he's an imaginary friend, his feet are the cinderblocks). He also loves gum, to the point where he'll even eat gum people have already chewed.
In "Cuckoo for Coco Cards", he offers Bloo a case of a complete set of Coco Cards in exchange for gum.
A mysterious imaginary friend addresses Bloo by his card trading name, "cardshockmasterb23," and states that Gumbo wished to speak with him.
Once Bloo gets to the basement, Gumbo promptly states his name and his height, and then presents a suitcase containing the entire collection of Coco Cards. Gumbo agrees to exchange this suitcase for all the gum that Bloo could find. Bloo proceeds to gather as much gum as he could from trash cans, underneath tables and bus seats, etc. He then brings all the gum back to Gumbo in a neatly stacked and shaped pile.
Gumbo examines a piece of gum and then starts to chew it. Gumbo then immediately shovels all the rest of the gum that Bloo found into his mouth and then proceeded to blow a large bubble. This bubble was so large that it acted as a helium balloon, allowing Gumbo to be lifted off the ground and into the sky (despite his feet being encased in cinderblocks.)
In Bad Dare Day, Mac dares Gumbo to eat Bloppy Pants' Mexican cooking, to which he refuses presumably because Bloppy Pants is a terrible cook. Gumbo last appears in Goodbye to Bloo attending Mac's going away party.