Thank you for your replies.
I have been able to create a gradient fill in a 3D document however the result is a poor looking building compared to the 3D view in which the building looks quite good.
The downside with the 3D view is that I can not put the sky gradient in or the earth Gradient in.
The other problem with the radial Gradient for the earth is that I can not make the circular pattern of the gradient oval to create a 3D look.
I am still interested in how the gradient fill was applied to a slab. This would be very useful.
Thank you.