I can't imagine why anyone would use canned gravy when gravy is so easy and fast to make from scratch. All the processed ones, whether they be in a jar, can or package taste metallic to me.
I think if the casserole was somthing I made more frequently than other dishes, I would make gravy and freeze it for use at a later date. After Thanksgiving, for example, if I had stuff to make more gravy, that would be a good time to bag some up in the quantity your recipe calls for.
M