The Garmin Nuvi GPS will automatically prompt you an option to switch to Simulator Mode when it is not able to contact with the satellites. Simulator Mode allows you to use the functions of the GPS, such as planning for a route to destination, even when you are indoor and cannot get satellites signal. Simulator Mode is very useful when you want to demo the GPS functions to others.
You can manually set the GPS into Simulator Mode, even when the GPS is able to get ssatellites signal, by going to Tools > Settings > System > GPS Mode and select "GPS Off". The GPS signal strength indicator on the top-left corner of the screen will have a red cross when you are in Simulator Mode.
From the Simulator Mode, you can switch your GPS back to "Normal" mode and start acquiring satellites signal by going to the same setting place.