Make a new map section?

Is there a way to make a sub-section of maps. For example there is the “Garry’s Mod” section with gm_construct and gm_flatgrass, and other valve games have their own sections, but I’ve seen custom sections somewhere before for example GMOD tower. How would I make one of my own?

It requires you to setup a gamemode file. It’s a lot more work than it should be.

How would i go about doing that?