Flood fill this selection with a pattern or solid color of your choice. This will become the left and right walls. This will be the layer for the ceiling and has to be behind the walls. Flood fill the selection with a color or texture. This will be the ceiling.
Flood fill with a color or texture.
You now have the floor. Simply use a rectangular gradient flood fill instead of a solid color for the ceiling and floor. I used the same colors for the floor, but reversed the foreground and background colors.