ΔE 12.2, fair: the same colour family, but not a straight swap. Semi Gloss Bright Gun Metal is noticeably darker, more saturated than Grey Knights Steel.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Semi Gloss Bright Gun Metal is noticeably darker, more saturated than Grey Knights Steel.
The same difference applies the other way round: Grey Knights Steel is noticeably lighter, more muted.
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 Grey Knights Steel on the colour wheel See Semi Gloss Bright Gun Metal on the colour wheel
All matches: Grey Knights Steel equivalents · Semi Gloss Bright Gun Metal equivalents