ΔE 15.5, loose: different enough that you should pick on purpose. Gunmetal is noticeably darker than Gun Metal.
✓ AK 3rd Generation equivalence chart lists these two as equivalents.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Gunmetal is noticeably darker than Gun Metal.
The same difference applies the other way round: Gun Metal is noticeably 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.
See Gun Metal on the colour wheel See Gunmetal on the colour wheel
All matches: Gun Metal equivalents · Gunmetal equivalents