Go to the main menu and type "gimmeall" without the "s. A window should pop-up and say you've just unlocked a cheat
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You have to get every trophy and unlock everything.
unlock them by doing the tasks in the notebook(2 button).
100% everything
As far as I know, the only way is to use the Unlock Everything cheat, which is I must note UNREVERSEABLE, is Green, Green.
Well it does depend on your NEVEC Level. If your NEVEC Level reaches 36 the handgun becomes available on the slot machine. If you use it enough after that the password for everything becomes available to use.