Chainmail vs Dark Gunmetal

How close are they?

ΔE 6.4, close: similar, with a visible difference side by side. Dark Gunmetal is a little darker than Chainmail.

Can I use Dark Gunmetal instead of Chainmail?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Dark Gunmetal is a little darker than Chainmail.

Can I use Chainmail instead of Dark Gunmetal?

The same difference applies the other way round: Chainmail is a little lighter.

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.