Load the game with the "hl.exe -dev -console -game gearbox" command line, then press ~ to display the console and enter one of the following codes:
God mode: /god
All weapons with full ammo: /impulse 101
No clipping mode: /noclip
Adjust gravity: sv_gravity [-999 - 999999]
Spawn indicated item: /give [item name]
Spawn indicated weapon: /give weapon_[weapon name]
Spawn indicated ammo: /give ammo_[ammo name]
Map select: 4/map [map name]