Offmap Artillery AP ammo is disable.

[PDF]

Hmmm, if you want to make bunker-busting scenarios just put the AP value you want for your gun in the "HE" row, in your example with Gustav change the "PEN 16:68" to "PEN 68:68", so your "HE" rounds will have power as if they were AP.
Anyway the shells of this monster should flatten anything they fall upon ...