i'm affraid it won't be that simple - you first need to point gpsd to some gps device, location of which is pretty much unknown in n900 (and even if it was, it still is probably kind of proprietary => unusable for gpsd)...

more bad news is that currently n900's gps seems to be a little inaccurate (if not useless - without internet connection)

the good news is, it should be quite simple to implement a GPS simulator based on the location API provided from nokia...