ΔE 17.7, loose: different enough that you should pick on purpose. Dark Gunmetal is noticeably lighter than Ironbreaker.
Can I use Dark Gunmetal instead of Ironbreaker?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Dark Gunmetal is noticeably lighter than Ironbreaker.
Can I use Ironbreaker instead of Dark Gunmetal?
The same difference applies the other way round: Ironbreaker 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.