Unfortunately, you are probably out of luck - perhaps a lesson learned about buying things (especially electronic things) at a pawn shop and not testing them at the shop. You could try to return it to the pawn shop.
Basically, your options are to use the PIN or be with 50 meters of the location that you first locked the phone.
This is from Garmin
Unlocking a Device when the PIN has been Forgotten
If you have Garmin Lock on your device and you have forgotten your 4 digit PIN number, you can unlock the device by returning to your security location.To unlock the device with the security location, it must be within 50 meters (about 164 feet) of the set location.
The security location is the location where you first enabled Garmin Lock on your device. Returning to the security location will unlock your device, provided the device is on and has acquired satellites.
Note: If you forget your PIN and your security location, Garmin Product Support cannot unlock the GPS.