From the manufacturer's website:
The Gun Metal spray is the latest installment in the successful line of Colour Primers from The Army Painter. The Gun Metal is a rich and dark metal and is nothing but the perfect basecoat for Knights and dark Sci-Fi & Fantasy. Especially vehicles will benefit tremendously from this spray – and a worn and battle scarred look is easily achieved by applying Warpaints colours onto the matt surface. We recommend Quickshade: DARK Tone or the Warpaints: Dark tone ink for the perfect shading.