Should cold turkey and roast beef be heated before putting on the gravy?
It would seem to me that the gravy could facilitate heating the meat without drying it out.

Pour the cold gravy over the cold meat, seal all with a cover or aluminum foil. Bake at 300F (150C) until the internal temperature of the beef is at least 140F (60C) and the turkey is at least 165F (74C). That's hot enough to satisfy the New York State Food Safety Division of the Department of Health. That's good enough for me even if the ultra-conservative FDA recommends 165F for the beef too (I'm finding conflicting information for pre-cooked commercial products).

I'm assuming your meat is safe to slice and eat chilled, but you are taking it through the "the danger zone", heating it up. So take all the way through to the other side (140F), and be aware of time. (2 hours in the danger zone, cumulative maximum).