Does GPS still work Without service plan? (WiFi only) *Help Please*

GPS works without anything. The iphone has a GPS receiver that works without anything else.

The maps on which the location is plotted often require an internet connection. I don't know about your app, but Tomtom and others store their maps on the iphone and no connection of any kind.

Either way, you should be able to get the maps via wi-fi - if your app does not store them locally on the iphone.
If it does store them locally, then you need no connection at all.

BTW, leave the SIM card that was active in the iphone. Without it you cannot upgrade or restore.

The GPS will work, but the only maps available will be those that are cached.

You might want to download one of the other navigation programs that include on-board maps and don't need a data connection, such as Navigon or TomTom.