You must have sv_cheats 1 and access to the console for this to work. Enter the following commands in the console in order:

Changes you to a level where you start with the super gravity gun - map d3_citadel_04
Switches you to the first level, replace d1_trainstation_01 with other map names if you want - changelevel d1_trainstation_01
Gives you the gravity gun again - give weapon_physcannon