GPS is independent to either of those. It has its own radio. It will give you long, lat, etc. even if you don't have either of those active.
Now if you are referring to getting map data, i.e. for google maps, either 3g/4g or wifi will work.
edit: I didn't realize that there were new rumors of a Wifi only version of the Xoom floating around again. If that was what you were referring to I don't know. There is no technical reason for it to not be on the wifi version as well though. The Xoom uses a S-GPS (simultaneous GPS) which means that it has its own front end (i.e. the receiver isn't shared with the 3g/4g radio) unlike some of the other ones. Because of this I would guess it would come on both.