Gosh, searching for Gumball and Penny fanfics feels like diving into a surprisingly deep corner of the 'Adventure Time' fandom.
Most fans seem to flock to the canon-adjacent stories. There's this author on AO3, I think their username is something like 'SassGathering,' who writes these vignettes about Gumball adjusting to royal life in the Candy Kingdom. The appeal isn't huge magical battles, but the quiet domestic comedy of a goofy dog-demon trying to navigate court etiquette while Penny tries to keep her dad from turning him into a gumdrop.
Other popular ones often rework earlier episodes. 'What Was Missing' obviously gets a ton of 'what-if' takes, focusing on the unspoken things between them. Honestly, a lot of the top kudos'd fics are gentle, character-focused slice-of-life stuff, which suits them. People aren't looking for epic crossovers here; they want that specific, sweet, slightly awkward dynamic from the show, just expanded. I keep hoping someone will write a really good mystery where they solve a crime together in the Candy Kingdom, but no luck so far.
I spend way too much time sorting by kudos, it's a problem. The ones tagged 'Fluff' and 'Hurt/Comfort' always have the most traction.