Various Cheats:
Start the game with the command line parameter hl.exe -dev -console -game gearbox. Now while playing you can press ~ to bring up the console.

It is also reported you can go to the console by pressing ~ then type in sv_cheats 1 instead of having to do the command line parameter above

Invincibility /god
Go to Map (insert map name for name) /map name
Get Item (Insert item name for item) /give item
Weapons & Ammo impulse 101
No Clipping & Flight /noclip
Give Weapon /give weapon ###
Gravity change Use numbers 10-800 for ## sv_gavity ##
Enemies ignore you /notarget

tems for Give code

ammo_556
ammo_762

Items for Give Weapon code

GRAPPLE
KNIFE
PIPEWRENCH
EAGLE
M249
SNIPERRIFLE
DISPLACER
SHOCKRIFLE
SPORELAUNCHER

Map Names for /map above

of1a1
of1a2
of1a3
of1a4
of1a4b
of1a5
of1a5b
of1a6
of2a1
of2a1b
of2a2
of2a3
of2a4
of2a5
of2a6
of3a1
of3a1b
of3a2
of3a3
of3a4
of3a5
of3a6
of4a1
of4a2
of4a3
of4a4
of4a5
of5a1
of5a2
of5a3
of5a4
of5a5
of6a1
of6a2
of6a3
of6a4
of6a4b
of6a5
of7a0

Play original HL levels with OpFor weapons:
All you have to do is start OpFor with the cheats on, start a new game or boot camp, and type "map c0a0" to start from the train, "map c1a0" to skip the train intro, or "map t0a0" for the hazard course. Now, just type "impulse 101" to get all the weapons including those from OpFor!!!!!

*WARNING- This can be slightly glitchy since the original HL wasn't made for the OpFor weapons