This mod allows you to set gamemodes per dimension. It is not needed client-side.
To set a gamemode for a dimension simply execute
/dimmode set <mode> <dimension>
where <mode> is the gamemode by name or number and <dimension> is the dimension ID (optionally prefixed with "DIM").
To toggle the config option for deleting player inventories when leaving a creative world do
The permission de.v10lator.dimensionalmode.command (defaulting to OP and CLI) is needed to execute the /dimmode command.