It's not realy cheat, it's some code for play at all multiplayer game mods in combat training. You have to press ² or on the key under Echap for open the console. The you have to write "g_gametype" and the code for the mod you want :
TeamDeathMatch : tdm
Free-For-All : ffa
Search & Destroy : sd
CaptureTheFlag : ctf
Domination : dom
Demolition : dem
Sabotage : sab
HeadQuarters : koth
GunGame : gun
OneIntheChamer : oic
SharpSooter : shrp
Stick & Stones : hlnd

So if you want to play gungame you write "g_gametype gun", and then you press enter and you write "map_restart" (It's make restart the game with the mod you want).

(Sorry for the English, I'm french.