ΔE 11.0, fair: the same colour family, but not a straight swap. Necron Compound is a little lighter than Gun Metal.
Can I use Necron Compound instead of Gun Metal?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Necron Compound is a little lighter than Gun Metal.
Can I use Gun Metal instead of Necron Compound?
The same difference applies the other way round: Gun Metal is a little darker.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.