ΔE 18.5, loose: different enough that you should pick on purpose. Blue Steel is noticeably lighter, much more saturated than Gun Metal.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Blue Steel is noticeably lighter, much more saturated than Gun Metal.
The same difference applies the other way round: Gun Metal is noticeably darker, much 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 Gun Metal on the colour wheel See Blue Steel on the colour wheel
All matches: Gun Metal equivalents · Blue Steel equivalents