ΔE 8.0, close: similar, with a visible difference side by side. Iron Hands Steel is a little lighter than Gun Metal.
Can I use Iron Hands Steel instead of Gun Metal?
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Iron Hands Steel is a little lighter than Gun Metal.
Can I use Gun Metal instead of Iron Hands Steel?
The same difference applies the other way round: Gun Metal is a little 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.