ΔE 15.5, loose: different enough that you should pick on purpose. Gunmetal is a little darker, more muted than Decayed Metal.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Gunmetal is a little darker, more muted than Decayed Metal.
The same difference applies the other way round: Decayed Metal is a little lighter, 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.
See Decayed Metal on the colour wheel See Gunmetal on the colour wheel
All matches: Decayed Metal equivalents · Gunmetal equivalents