Did you set the object mode correctly? For ground guns, remember the following (From the terrain Editor Manual):
The Value of the Object Data determines what kind of ground gun it is:
(0) 20mm
(1) 20mm Quad
(2) 37/40mm
(3) 88mm
(4) Light Machine Gun
(5) Heavy Machine Gun
So, in obeject editor mode, select your ground gun, then open its object properties, and make sure Object Data is set to 3. That should set it.
SD