Well it seems to me that it would be difficult to place that many pieces of ammunition within 10 feet of each other considering their size. On top of that, in order to carry it around, your player would clearly need a bag of holding or a portable hole or some such, but retrieving a single piece of that ammunition from the container takes a move action. So unless she spent 5 rounds doing nothing but taking out each piece of ammunition and placing it on the ground, she'd only be able to launch one of them per round. Of course, that's if you ignore the fact that an ordinary backpack couldn't hold gate breaker ammunition which would make retrieving the specific pieces of ammunition a full-round action - assuming other things are in the bag as well. You might rule that it's a full-round action anyway considering the size of the ammunition.
I suppose that your player could simply turn a bag of holding inside out dumping her ammunition out in front of her speeding up the process. However, you might not classify them as weapons so much as logs at that point due to the haphazard arrangement of the ammunition. Depending on how you rule it, a single blast of force might not orient the business end toward your enemies if they weren't pointing in the correct direction in the first place. If this were the case, the ammunition would only deal 1d6 points of damage as if it were a solid object weighing close to 25 lbs. If you do rule, however, that the violent thrust function of telekinesis spends the energy over the course of the action orienting and launching the ammunition then it seems completely viable to make all ten attacks in a single round. She would have to have a caster level of 12 to do that though as they are 30 lbs. each and telekinesis only increases the limit by 25 lbs. per level.
The ammunition would deal 3d6 damage as if a gargantuan dagger as Sowde has calculated because you are using gate breaker ammunition, not the gate breaker itself. The 6d8 damage is caused by the act of firing the bolt out of the gate breaker ballista itself, not by virtue of being gate breaker ammunition. Same reason regular bolts deal 1d4 damage instead of 1d10 as if fired from a heavy crossbow.
Note that if she used this attack against anything with DR, it would be severely limited in damage potential. An average hit against a creature with DR 5 would do 6 damage and DR 10 would mean only 1 damage with a damage ceiling of 8 barring critical hits. At 11th level, mages have decent access to defensive spells. Stoneskin would work and protection from arrows would be a cheap block as well. Even if all ten attacks hit, average damage would only wipe out most of the shield or do minor damage to the mage. In addition, at that level most creatures can be expected to have some DR, high AC, or both rendering the effectiveness of this attack relatively low. I wouldn't be too worried about allowing this in my campaign considering that it is really only a threat to relatively unprotected enemies.
She'd be much better off with a fireball or a cone of cold or something, but this would bypass any resistance a target might have so it could be viable. But then, against the right target at the right time, anything is viable.
