I would find another daycare. To leave the dogs, even for a second, unsupervised is wreckless.
However, as for the incident, since no one say what really happened it's going to be impossible to figure out. I wouldn't find either dog faultless and if the lights were out and Gauge jumped the other dog, the other dog might have panicked. That doesn't mean that dog is aggressive (per se)
Regardless, that's not the point, if there is no one in the room, it's just dangerous all around - incidents can happen in the blink of an eye. I wouldn't care so much if the other dog is still there (since when they found Gauge he was on top of the other dog so he very well could have had some part in what happened as far as instigating it). What I would care deeply about are their policies and procedures. Might be a good idea to find a new daycare.
Where are you located?
Murdock - GSD
Virgil, CGC - GSD
Tara, Reagan, Madison, and Jefferson - Aussies
The birds: Dante - CAG / Nero and Sunshine - Budgies