I won't bore you with the scientific data(!) but radio waves are funny things - mileage will always vary depending on so many different factors. Radio gets absorbed and attenuated by obstacles in the way so ultimately you don't want to force the signal through too many walls or floors.
If you're networking on one floor only, then getting the gateway up high on that floor will help a bit (or a replacement antenna mounted up high). That makes for a better line-of-sight from PC's to gateway (if the gateway is transmitting at desk level, then the signals have to travel through all those desks, and each desk is attenuation).
If you're working across multiple floors then the best system would be to have a gateway on each floor hard-wired to a central hub.
PS If you want to go really mad, do a google search for "pringle can antenna" - some nutcases have made 802.11 work up to about 15 miles with those(!), but that's just for curiousity and has no practical value....