You do it in the command line to your server? That's where you set the default gamemode.
Here is an example of one of my servers.
C:\GMod Servers\stranded\orangebox\srcds.exe -game garrysmod -ip 216.245.209.83 -port 27017 -console +gamemode sgstranded +hostname "g4p.org | GMStranded 13 Server #2 - Dallas" +map gms_stranded13_worlds -tickrate 66 +sv_scriptenforcer 0 +maxplayers 18 +sv_kickerrornum 0 +rcon_password **snipped**