ΔE 21.3, loose: different enough that you should pick on purpose. Dark Gunmetal is noticeably lighter than Polished Silver.
Can I use Dark Gunmetal instead of Polished Silver?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dark Gunmetal is noticeably lighter than Polished Silver.
Can I use Polished Silver instead of Dark Gunmetal?
The same difference applies the other way round: Polished Silver is noticeably 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.