This show works on both an adult and a kid level.
Gumball Watterson is a typical twelve-year-old boy (who is a blue cat) facing typical junior high school issues. His crush is a peanut with antlers. His teacher is a cranky old monkey/ape named Miss Simian. His principal is Cousin Itt from the Addams Family. His guidance counselor is a hippie cloud guy. He has to deal with a bully (a T-Rex) and a snotty girl who is a cloud. His brother Darwin is a pet goldfish that grew legs. His sister Anais is a genius four-year-old bunny. His dad is a big pink rabbit and his mom is also a blue cat. All of this is accepted at face-value as normal and unremarkable to the extent that all the visual wackiness and mix of 2D and 3D animation and live-action backgrounds become merely that, the background to the wackiness that Gumball and Darwin get in to and out of.
Kids can relate to the usual shenanigans one can expect from twelve-year-old boys in junior high. The tight writing keeps the action and the jokes coming, often from unexpected directions (such as a show about a lazy contest suddenly going into a "Sugar Rush" song with dancing sugar cubes massaging a heart in a ribcage).
What sets this show apart though is although the Wattersons are one of the most dysfunctional families ever shown on any show on TV, they truly love and care for each other. (One episode even deals with this very subject!) Richard, the father, a typical TV fat stupid guy, sits at home all day in his underwear playing video games. His incredibly competent (and may I say hot) wife Nicole works long hours at the rainbow factory to support them all. His stupidity and the boys' antics set up situations that are resolved by Nicole's forceful personality and Anais' wisdom.
There is just as much (if not more) adult drama (and references) going on here to keep the interest of us older viewers. And just look a the Radar page to see all the adult humor that gets slipped in.
So to sum up, this creatively written and made show keeps the whole family entertained on all levels!