Every other plugin has better /gamemode cmds.

At least essentials has /gamemode [letter] or /gamemode [word]

And don't say it works because this has ZERO purpose. (Basically what ShaneBee said)

this is a terrible script, It overrides the vanilla gamemode command and uses some dumb numbers circa 1.12 and breaks all functionality of the vanilla gamemode command.
on top of that its poorly written and serves no place on SpigotMC