ΔE 17.1, loose: different enough that you should pick on purpose. Gunmetal Grey is noticeably lighter than Necron Compound.
Can I use Gunmetal Grey instead of Necron Compound?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Gunmetal Grey is noticeably lighter than Necron Compound.
Can I use Necron Compound instead of Gunmetal Grey?
The same difference applies the other way round: Necron Compound 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.