The description for the Gutbuster Ball round does not specifically say that it Destroys Buildings, so it does not. However, it does say the following.
Quote:
Anything under the template is automatically hit and must save at a -4 modifier. After the location of the template is determined the cannonball will roll 4 D6 cm (use the special rolling ball template for this) along the line of fire (directly away from the Gargant). Any model in the ball’s path will be struck automatically and must save with a -4 modifier.
I can see how this could be interpreted to mean that the Ball round could have Damages Buildings, but it should not have Destroys Buildings. That said, even if a building survives being hit by a Ball round, the Ball round would keep moving to the full rolled distance. Basically, the save determines whether the Ball hit any essential support structures.