ΔE 18.8, loose: different enough that you should pick on purpose. Iron Hands Steel is noticeably darker than Gun Metal.
Can I use Iron Hands Steel instead of Gun Metal?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Iron Hands Steel is noticeably darker than Gun Metal.
Can I use Gun Metal instead of Iron Hands Steel?
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.