ΔE 19.5, loose: different enough that you should pick on purpose. Dark Gunmetal is noticeably lighter than Adamantium Black.
Can I use Dark Gunmetal instead of Adamantium Black?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Dark Gunmetal is noticeably lighter than Adamantium Black.
Can I use Adamantium Black instead of Dark Gunmetal?
The same difference applies the other way round: Adamantium Black 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.