Rechargeable Batteries - Can I use them in my GPSr?

Yes you can with a couple of points to note:

bulletRechargeable batteries are slightly larger in diameter then normal batteries. For tight fitting battery compartments like the Garmin 45, you may have trouble removing them.
 
bulletGPS units do not have recharging circuity within them so don't expect them to be recharged if you plug in an external power source.
 
bulletThe battery indicator will not read a true reading of the state of the batteries. When rechargeables are fully charged they may still read  only 90% on the battery gauge. Also they battery gauge will tend to read high, maybe 50%+  until about the last 10% of the batteries charge.  This is a characteristic of the way in which rechargeables discharge  not a fault with he GPS unit.