Put it in an RF chamber and keep another GPS receiver outside the chamber some distance away to make sure it doesn't lose lock on the real satellites. That's your leakage canary.
We had one of those in an underground parking garage for autonomous vehicle testing at a previous job, but it was a naturally really well shielded room, and it was just repeating surface signals so no one would complain.
Don't go spoofing or broadcasting your own GPS signals unless you have a decent legal team behind you.