Gun Metal vs Orgoth Bronze

How close are they?

ΔE 21.8, loose: different enough that you should pick on purpose. Orgoth Bronze is a little lighter, much more saturated than Gun Metal.

Can I use Orgoth Bronze instead of Gun Metal?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Orgoth Bronze is a little lighter, much more saturated than Gun Metal.

Can I use Gun Metal instead of Orgoth Bronze?

The same difference applies the other way round: Gun Metal is a little darker, much more muted.

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.