Install seperate GPS at all user locations or a central GPS for Time distribution ??

You don't say what kind of time accuracy you require, is 1 second OK?, or are you looking for mili-micro-nano second accuracy?

 

Do you have the means to supply micro-second accuracy to the 20 useres via "a central GPS receiver and a communications link"?, if not you don't have a choice, you would have to go with a separate GPS receiver at each location

 

This is beginning to sound a bit like problem set by a communications course lecturer.

If you can extract nano second accuracy from the centrally installed GPS receiver, why would you only get micro second accuracy from the GPSs you would supply to every one of the 20 users?, cheaper GPS units perhaps?, if you installed the same nano second GPS unit in all 20 sites you would get nano second accuracy everywhere wouldn't you?

I think this is a question about propagation delay, when sending nano second accurate time from a fixed location to 20 fixed locations it would be possible correct for any propagation delay because it wouldn't vary too much, however, trying to correct for propogation delay from multiple moving satellites (who's distance from each user is constantly changing), may be harder to achieve