The vast majority of the club use some form of a Garmin GPS devise. Most popular is the BMW Nav V and Nav VI, which are made by Garmin. Some use other non-BMW’ed Garmin devices.
The BMW units are quite nice as they interface with the bike, allow you to control things from the WonderWheel. They are expensive ($900?), but you can occasionally find a used one, like the one Craig G just sold for $400.
It would be best to start a new thread under the GPS Forum and others who are more knowledgeable on GPS Apps may chime in.