I have seen them at weddings but never had a desire to go eat what other people were sticking their fingers in.
Also I don't see how it keeps the food at a proper temperature to not be a swirling bacteria pool.
There is no possible way for the fountain to keep the food at a proper holding temp above 135°f. However, you can keep time/temp vulnerable foods for up to 4 hours before any real concerns for bacteria growth. But I recommend you don't let things sit for more than 3 hours. But if the gravy fountain is being emptied and sanitized within that 4 hour window, then it will be fine.
Although, I wouldn't trust people to keep their grubby fingers out of there.
I think the obvious safe choice here is to just make a few thousand pounds of gravy and only let the gravy run through the fountain once. Whatever isn't consumed on its first trip down goes directly down the drain. Solves both the bacteria and grubby finger problems!
As a bonus, it could drain into a large plastic swimming pool for a gravy-wrestling match.
Food fountains are grossly unhygienic.