Excellent question, doujinshidude! These two Japanese gay slang terms both describe big men (and gay artwork that depicts big men). Their meanings have considerable overlap, and a lot of our favorite gay mangaka draw both gachimuchi (ガチムチ) men and gachidebu (ガチデブ) men, sometimes in the same illustration.
The vocabulary used in Japanese gay art tends to invite a range of subjective interpretations, just like queer nomenclatures all over world. Queers seem to understand on the versatility of language better than most— perhaps out of a need to escape the rigid confines that heteronormative language places upon our self-definitions. So bearing in mind the ever-shifting nature of queer language and the shortcomings of my own Japanese skills, I’ll do my best to break down the broad distinctions between gachimuchi and gachidebu!
Gachimuchi essentially means muscular with a slight coating of fat. Picture “a middle-aged Japanese professional wrestler”—a colorful metaphor provided by the NicoNicoPedia definition. The word is a portmanteau of gacchiri (がっちり), meaning “athletic, solidly built,” and muchimuchi (むちむち), meaning “plump, voluptuous.” In anime and manga circles, muchimuchi classifies female characters with implausibly over-sized curves—i.e. giant boobs. Similarly, gachimuchi men boast implausible curves and extremities of their own—Jiraiya’s hyper-hypertrophied muscles and Senga Migiri’s gargantuan cocks, for instance!
Gachidebu is just like gachimuchi, only fatter. “Sumo wrestler” size, according to the Pixiv Encyclopedia. Instead of “muscle-voluptuous,” gachidebu dudes are fully “muscle-fat.” The difference is often visible in the belly, as in the above example from a recent doujinshi by Takaku Nozomu. Fat abs? Gachimuchi. One-pack? Gachidebu. If the character is fat but not muscular at all, he’s simply debu (デブ).
In contemporary American gay parlance, a gachimuchi character might be considered a “muscle bear”—though gachimuchi doesn’t imply hairiness the way "bear" inherently does. Gachidebu could be translated as “muscle chub,” (hey, Google found a Tumblr for that!) and “chub” for debu.
Of course, these anglophone corporeal classifications are just malleable and subjectively defined as their Japanese counterparts. There’s no entry for “muscle bear” in the Oxford English Dictionary, as I presume “gachimuchi” is omitted from the standard-bearing 14-volume Nihon Kokugo Daijiten. Isn’t that exciting? New language, fresh out of the oven! Play with it while it’s still warm.