To insert the following codes, enable the console from the in-game options. Press ~ and type in "seta thereisacow 1337", pressing Enter thereafter. From here, type "spdevmap bog_a" and press Enter again. Then, you can insert the following cheats.

jump_height # - Adjust gravity (39 is default)
noclip - No Clipping
notarget - Dumb AI
r_fullbright - Brighter features
timescale # - Adjust the time (1.00 is default)
ufo - No Clipping (II)