To download goTenna’s offline maps, choose “Location” from the Side Menu, then click the gear icon in the upper-right corner of the map screen. Next, select “Downloaded Maps”, and 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 maps are free. You do require 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, you should download maps when you're on wifi.