I'm no expert and have no idea on how to light down a metalic paint. But I have used three diferent gunmetals paints:
Tamiya gunmetal (X-10).- it's an acrylic and is lighter one i have used.
MM Gunmetal.- It's the darkest one. You should always thin this one.
MM Gunmetal metalizer.- It's somewhat lighter than the none metalizer. You can try to hand brush it, but you have to do it twice since it's already thinned to airbrush-ready consistency.
Here are a couple examples: the machinegun on the Hummer was painted with MM gunmetal (non-metalizer) enamel, while the machinegun on the stug was painted with the tamiya acrylic.
http://img182.exs.cx/img182/1455/img00960at.jpg
http://img68.echo.cx/img68/2421/039hd.jpg