If I want to use the app Layer ( Augmented Reality Browser: Layar ) which uses my GPS do I need a data plan?
^^^ Not true, he could be willing to shell out the $$$$ to buy one outright.
Then the question is would Verizon let you activate it still without the data plan!!! Also one has to ask why you wanting the phone and no data plan???
No you do not need a data plan for GPS to work. Data is not sent through 3G it get GPS functionality. Just like Garmin does not need a plan. GPS is free for all to use, one of the few things that is free once you have a device.
Edited to say: I used the GPS a few times with no towers around.
You can certainly buy the Droid straight out, but as soon as you try to activate it (even if you do it online or on the phone), Verizon will tack on the $30 data plan to your bill.
GPS needs the data plan to show the map, but not to find your location. Without data, you will see a blank map.