1. Gutbuster ball shot. Does it destroy buildings? In the list of building destroying weapons it says no, but it feels like it is one of the most powerful weapons in the game and would be powerful enough. In fact it feels kinda weird that it wouldn't.
2. Do buildings stop a ball round or does it pass through? I feel that if the ball just hits a series of external and interior walls it would smash through no problem. But if it hit a serious structural section it could maybe be stopped. So which is it? Maybe if it destroys the building the shell stops, and if not it just passes through the walls and continues.
3. If buildings can't be destroyed and/or stop gutbuster shells, then do low buildings stop the shell or can you say it is fired over the top? The way the gut buster wreaks a path of destruction down its entire course implies that it's skimming along the surface. But if that means low buildings stop gutbuster shells, then that gives gargants a massive disadvantage against any enemy Titans standing on the other side of the low building who get to fire back with their full arsenal.

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.

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.
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.