This tutorial will cover how to add a custom gametype to the Set Game Mode list in Custom Games. This is only for Multiplayer mods.
In your core_mod.zone, add the following:
In your mod folder, add the following folder structure: gamedata/tables/common/
From the raw files, copy the default mod_game_types.json file from Call of Duty Black Ops III\share\raw\gamedata\tables\common to your mod's gamedata/tables/common/ folder.
It should look like this:
Change MPUI_TEAM_DEATHMATCH and MPUI_TEAM_DEATHMATCH_CAPS to localisation keys for your mod. And change "tdm" to the name of your gametype.
For example, in my Onslaught mod I created a onslaught_menu.str file in english/localizedstrings, with the following:
And in the mod_game_types.json for the mod I did this:
Once you have done that, re-compile and you should see this ingame: