After the follow Adding a Map section, you will understand here quickly.
By meaning game modes is simple. Single player, Co-op, Deathmatch and Team Deathmatch.
Adding a game mode on existing map is like creating a map.
After the creating a map just copy the map folder and and change its MapData.json file to make it a new game mode.
JUST DON't FORGET TO ADD A LANGUAGE ID FOR THE NEW MODE, OR YOU WILL SEE TWO SAME MAP NAMES ON GAME CLIENT
teamdata:0 is Deathmatch
teamdata:1 is Co-op
teamdata:higher than 1 is Team Deathmatch
minplayers:1 is Singleplayer