The variable physcannon_mega_enabled turns the super gravity gun on and off. To turn it on, you type the following at the console:
And to turn it back off, you type this:
You can do this at any time during the game, but I feel I should mention that I encountered a serious problem with the super gravity gun and the buggy that you drive on Highway 17. You see, the super gravity gun could pick the buggy up, but the physics engine couldn't handle it, and the game would get really slow, and then freeze up. I'm not sure if this happened for everyone, and it may not be a common issues since I'm on Vista x64.