GSM is pulling information from the GlobalSubscriber and GlobalLocation tables in UnityDB on the local SQL server - so the problem doesn't stem from GSM, but with the directory monitors not populating those tables. You can check out those tables yourself to see if users from other boxes are there or not.
Is this installed into a child domain or what's it's relationship to the other boxes on the network? Likely we're not seeing the users/locations for the other Unity boxes in the directory for some reason or another - that's where to start. Can you see them in ADUC off the new Unity box?