The iPad comes either wifi only capable or wifi + cellular capable. The GPS chip only comes with the wifi + cellular models.
There are basically two types of navigation apps. Apple Maps, Google Maps, and many others work primarily from cellular to download information such as maps on the go (not stored in iPad) and satellite. Cell towers supposedly work faster in locking in a location. The GPS is considered an assist. You do need a cellular plan in order to use them for navigation but you can also use the maps to read and plan if you have a wifi location similar to the way you would on a computer at home.
The other kind of navigation apps similar to stand alone units like TomTom, Garmin, Navigon, Magellan, etc. have their own data of maps in the iPad. They work without any cellular connection whatsoever and they work well. And you don't need cellular for location. The GPS can do it all without need of cell tower location or even a cellular plan. What it can't do without a cellular plan is access the Internet obviously for extended data some of the programs feature but you are no worse off than a standalone GPS unit you might buy.
The standalone apps that can use the GPS unit without needing a cellular connection generally are not free and can generally cost from $30 to $60. But then you have everything a TomTom unit can do for that price if you already own a wifi + cellular iPad.
Last year I drove from Minnesota to Alaska and all over Alaska. I never had cellular once I entered Canada for over 10,000 miles of driving and the TomTom North American app I bought for $30 never failed me. In some areas there were no cell towers for miles even if I desired to pay and use them.
What you have to understand is some GPS apps can be used for navigation without a cellular data plan but cannot be used unless you own a wifi + cellular iPad because the GPS chip is only in those kind.
