Dark Steel vs Gunmetal Blue

How close are they?

ΔE 25.9, loose: different enough that you should pick on purpose. Gunmetal Blue is noticeably darker, more saturated than Dark Steel.

Can I use Gunmetal Blue instead of Dark Steel?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Gunmetal Blue is noticeably darker, more saturated than Dark Steel.

Can I use Dark Steel instead of Gunmetal Blue?

The same difference applies the other way round: Dark 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.