Parable tiem:
>be me
>come up w/ idea for Victorian GURPS campaign (Sherlock Holmes-ey, mysteries and cults)
>"that sounds great anon, I'll run it" -my friend
>show up to the game w/ Dr. Linus Flaversham, a psychiatrist who uses hypnotism to treat patients and is hiding an obsession with the occult
>DM has taken the idea and is now running a Steamshit game with gay-ass magyckal machine guns and Black Ops government missions
>play one session and never again
>game has become the hallmark of shitty bait-and-switch for our circle
This is what you are doing. It is one thing for a game to take unexpected turns (oh look, you return to your castle to see it has been captured by bandits); it is another thing to lead your players false, and go so far as to create fake papers to mislead them. They think they are going to be playing a modern shooter. They are excited to play a modern shooter. You never had any intention of making this happen.
They will not be happy.
To make this work, run a few sessions in the modern setting before you pull out the portal. At least that way they don't feel cheated and may actually see your big twist as an actual twist.