I've been at the church at 6th & M (United House of Prayer) several times for their cheap, home-cooking cafeteria (fried fish, fried chicken, okra, macaroni & cheese, etc.), and I never saw a "Daddy Grace's Kitchen" anywhere.
A chowhound once mentioned that there was a separate place to eat in the very back of the building, I walked all around the back parking lot, but all I could see were dumpsters & locked doors to the cafeteria's kitchen. No signs, no open separate dining place, even for carry-out.
By the way, the House of Prayer just south of the Shaw/HU Metro station also has a cafeteria, but it's much smaller than the one at 6th & M. Fewer choices of eats. Price are the same, I believe. You pay around $9 for an entree, 2 sides & some bread.