ΔE 13.7, fair: the same colour family, but not a straight swap. Plate Mail Metal is noticeably lighter than Gun Metal.
Can I use Plate Mail Metal instead of Gun Metal?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Plate Mail Metal is noticeably lighter than Gun Metal.
Can I use Gun Metal instead of Plate Mail Metal?
The same difference applies the other way round: Gun Metal 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.