Nuln Oil gloss is fantastic for painting metallics, especially gun-metal coloured metallics. It allows you to add depth to your work without losing the shine. I have personally used Nuln Oil Gloss over Leadbelcher with amazing results. The only thing you need to worry about is pooling, as the pools dry both darker and more glossy, but that's a consideration for all Shades.
Overall, a great product I recommend to anyone who needs to paint metallics. 10/10