works by using either a 4 digit pin code or by taking the unit back to a preset security location. Although Garmin warn that this should NOT be the user's home address, that's what it normally is. If you know where the last owner lived then you can try going to that location to see if it unlocks, but if you knew that piece of information you could just ask the last owner...
It came from a pawn shop though, so it's very unlikely that you know who the last owner was or where he/she lived.
There is no other way to unlock your unit without sending the unit back to Garmin. There's a charge for the service and you'll be asked to provide evidence that you own the unit; in practice it's going to be cheaper to buy a similar brand new Garmin model. All I can suggest is that you try to get a refund from the pawn shop (unlikely!) or sell it to another unsuspecting person.
Sorry, but Garmin Lock is deliberately difficult to overcome otherwise it wouldn't be much use as an anti-theft deterrent.