ΔE 6.4, close: similar, with a visible difference side by side. Grey Knights Steel is a little darker than Gun Metal.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Grey Knights Steel is a little darker than Gun Metal.
The same difference applies the other way round: Gun Metal is a little lighter.
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