No - it is not true. The GPS on a 3G iPad works whether or not the cellular connection is active. However, if you're using the iPad's Maps app then this downloads mapping data in real time and that would require a WiFi or 3G connection to download the map data. But there are many other mapping apps that download the map data while you're at home, say, and then you would not require a cellular connection.
I have an iPad1 with 3G and often use this in areas where there is no cellular connection possible (no coverage) and the GPS works just fine.
Tim