Place +sv_defaultgamemode <gamename> in the command line that starts your server, and/or in /cfg/server.cfg without the +
Not in any ULX file.
Gmod has to know what gamemode to load before any other addon loads.
The ULX gamemodes folder is only for if you wish to set different options than what runs in our /configs/server.ini when the gamemode loads.
[Edit] - Moved. A Server how to, Not a ULX how to