Motherlode vs Dark Gunmetal

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Dark Gunmetal is a little darker, much more muted than Motherlode.

Can I use Dark Gunmetal instead of Motherlode?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Dark Gunmetal is a little darker, much more muted than Motherlode.

Can I use Motherlode instead of Dark Gunmetal?

The same difference applies the other way round: Motherlode is a little lighter, much more saturated.

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.