GPS will not work indoors because the satellites cannot be "seen" by the receiver.
You could make a system that behaves like GPS and tells you which room in a large building you are in. The first step though would be to define clearly what you want to achieve.
For example you could have some sort of transmitter at each door identifying the door.
As a user passes through the door his receiver would learn its position.
This involves lots of transmitters though, plus a map of the building identifying the location of each transmitter. It also only tells the general location of the user, not his position within a room.
Buildings may already have a security system with passes to get into different zones. In that situation it may be possible to interface with the security system (in the unlikely event you can get access to it) and have it tell the user which zone he is in.