ΔE 16.2, loose: different enough that you should pick on purpose. Natural Steel is noticeably darker than Semi Gloss Bright Gun Metal.
Can I use Natural Steel instead of Semi Gloss Bright Gun Metal?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Natural Steel is noticeably darker than Semi Gloss Bright Gun Metal.
Can I use Semi Gloss Bright Gun Metal instead of Natural Steel?
The same difference applies the other way round: Semi Gloss Bright 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.