GPS will work without the SIM card or a plan. However; most map apps use the internet to download the maps themselves, so without a connection 3G or WiFi, you may find yourself accurately placed on a blank page.
But, most of those same map programs will catch map data when they do have a connection. That means if you look over your route and/or destination while you have a connection, you may be good.
And, there are map programs that come with or can download maps on the iPad. They are generally much more expensive.