To purchase some Naval Guns on land map. First you need to set up a coastal map (

Like No.20 Coastal City,SE Asia ) and choose a force with naval gun (

like USA ) , then purchse some naval guns. After that, click the "view map" button to enter the map screen and change the map to a land map.
Yap, you`ll be able to bombard your enemy with naval gun on desert or in jungle

And also the formation for purchase list is messed up as some formations in Artillery like "Light Battalion" and "NG Lt Bty" show the icon of naval gun and some other formations are totally unusable.
So we should go into the "Allied" screan and choose any of the forces, go into its "Artillery list", then "Allied" back. Yap, you can now also buy some land off-map artillery.

In PBEM game, only the host could do this ^^