Desert, Frontier Towns, No Central Government
Colonized Land That Never Stopped Being a Frontier
Independent Towns, Not a Unified World
Gunsmoke isn't governed as a single civilization, it's a patchwork of independent frontier townships scattered across a mostly uninhabitable desert, each one running on its own rules, its own law enforcement or lack of it, and its own relationship to whatever Plant is keeping it alive. That fragmentation is exactly what lets a bounty as absurd as Vash's stay chaotic instead of centrally enforced.
Why Everyone Speaks English
A small but genuinely fun worldbuilding detail worth having on record: creator Yasuhiro Nightow has stated on the record that English is simply the language spoken on Gunsmoke, an intentional choice rather than a translation convenience, in keeping with the show's spaghetti-western DNA.