ΔE 24.6, loose: different enough that you should pick on purpose. Iridescent Stainless Steel (Coarse) is noticeably lighter than Gun Metal. They are different kinds of paint: Army Painter Gun Metal is a metallic and Golden Iridescent Stainless Steel (Coarse) is a pearl paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Iridescent Stainless Steel (Coarse) is noticeably lighter than Gun Metal. They are different kinds of paint.
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.
See Gun Metal on the colour wheel See Iridescent Stainless Steel (Coarse) on the colour wheel
All matches: Gun Metal equivalents · Iridescent Stainless Steel (Coarse) equivalents