Alter game files at your own risk and always make backups. Navigate to the game folder, find the user.cfg file and open it in a text editor. Change the following lines and save cahnges for the desired effect.

Change g_global_god off to g_global_god on
and change g_god off to g_god on for God Mode

Change g_unlimitedammo off to g_unlimitedammo on for unlimited ammo.