ΔE 10.0, fair: the same colour family, but not a straight swap. Grey Knights Steel is a little lighter than Gun Metal.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Grey Knights Steel is a little lighter than Gun Metal.
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.
See Gun Metal on the colour wheel See Grey Knights Steel on the colour wheel
All matches: Gun Metal equivalents · Grey Knights Steel equivalents