From your previous comment, it sounds like your experience may have been from a while ago? In 2022, it is fairly trivial and cheap: https://github.com/osqzss/gps-sdr-sim
I can not ;^) personally confirm that this works with a HackRF, which is like $300, but probably also with any other reasonable tx-capable sdr.
Trying to set up an alternate 3d volume of GPS space sounds very difficult.
But broadcasting a loud signal that tells everyone in range that they are at the same exact point doesn't seem too hard to me. Couldn't that even be as simple as replaying a single-antenna recording taken somewhere else?
Doesn't work well with some receivers that cache data from the real network and stay locked onto the much weaker real signal. But works with most receivers.
