GarminPilot does not need a 3G/LTE connection.
You will have to download charts and documents and store them locally on the device.
Some things to consider: Every time you install an update, turn off wifi (put device in airplane mode), then start GP. Sometimes the updates will want to connect back to Garmin to finish resetting the system. That's not something you want to find out when you are in the air and away from the FBO wifi.
If you don't install the charts, when you fly off the edge of the charts you do have, the chart will simply show as a gray checkerboard. All the GPS information will still work, but there won't be a chart to display. By putting the device into airplane mode, loading your course, then zooming in on the charts you'll find out if you are missing anything.
edit: on the chart download page you'll see a map of the US and all the charts overlayed. You can select which charts you want to download, so you don't need them all. Some charts will overlap - like a TAC and Sectional - so make sure you get them both. When you are on a sectional and then zoom in far enough, the TAC will be displayed. And remember to download whatever chart supplement/AFD for your area, too. Once you do that, the system will let you know when the charts you have will need to be updated.