Choose “Location” from the Side Menu, then click the gear icon in the upper right corner of the map screen. Select “Downloaded Maps”, then select “Available” on the “Map Downloads” screen. Choose either by Country (e.g. France) or by Country > Region (e.g. United States of America > California).
All standard maps are free and available for the whole world. You will need a data connection to download the maps, so plan to do so before you go off-grid! And because some of these maps can be big, wifi is recommended.
(Note: By upgrading to goTenna Plus you can access offline topographic maps to get richer detail of your surroundings, which is particularly useful when enjoying the outdoors.)