Both are young heroes placed in unique worlds (In the MHA universe, 80% of Earth's population have superpowers, and Elmore is a haven for literally anything possible) who were originally created with somewhat "darker" intentions (In a 2008 one-shot, Izuku was supposed to remain Quirkless, and during development, TAWOG was a series about rejected cartoon characters going to a miserable school), although overtime, their creators (Kohei Horikoshi and Ben Bocquelet) started taking inspirations from their idols (Stan Lee and Bill Watterson), which gave us the franchises we know today.
Initially considered "normal" (Izuku was Quirkless, and Gumball discovered he was the only one at school without a special power) and distraught over it, they eventually found their true power thanks to their wise, mentor-like figures (All Might inherited One for All to Izuku, and Darwin helped Gumball learn his body was malleable).
Although their personalities are polar opposites (Izuku is courageous and selfless, and Gumball is lazy and selfish), they never forget to keep their optimism around even in dire situations (Izuku knew he could take on Muscular despite still being a noob with OFA, and Gumball tried to lift his family's spirits when the show's budget was running out). Most of these situations lead to them having to face the risk of death, especially due to their high determination (Izuku damn well nearly destroyed himself in the villain war, and Gumball almost died just so Tobias could slap his ass), but due to their personalities and pain tolerance, they manage to come out fine (Izuku always saves people with a smile just like All Might even if it means breaking his bones, and Gumball tends to underestimate his opponents only to survive at the end thanks to his regeneration).
Accompanied by the stoic, intelligent one with little to no social skills (Todoroki and Anais), the honorable friend who always takes things seriously (Iida and Darwin), and the bubbly, sensitive love interest who can become determined and intimidating at will (Ochaco and Penny), life isn't always perfect for them, as both often have to fend off the arrogant, immature villain (Shigaraki and Rob) who are highly cunning, have tall, slim bodies, and feel as though the world needs to revolve around them (Shigaraki wants hero society to be destroyed in his own hands, and Rob doesn't want Gumball to be the star of the show).
However, these villains were revealed to have been misunderstood - somewhat (Shigaraki was abused by All for One so he could hate heroes, in fact Izuku heard a crying child deep within him, and Rob tried to transform everybody into humans so he could warn them about the universe falling apart).
Gumball Watterson (The Amazing World of Gumball)
Izuku Midoriya (My Hero Academia)
