But the bureau was already gravy. What does gravy mean in this context?

In this context, "gravy" refers to something that is an additional benefit or extra reward beyond what is necessary or expected. It suggests that the bureau is already receiving more than its fair share of resources, profits, or advantages, implying a sense of surplus or excess. Essentially, it indicates that the bureau's situation is advantageous or profitable, often with an implication of ease or lack of effort involved in obtaining that benefit.