ΔE 23.8, loose: different enough that you should pick on purpose. Greatcoat Grey is noticeably darker, more muted, warmer than Stormfang.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Greatcoat Grey is noticeably darker, more muted, warmer than Stormfang.
The same difference applies the other way round: Stormfang is noticeably lighter, more saturated, cooler.
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 Stormfang on the colour wheel See Greatcoat Grey on the colour wheel
All matches: Stormfang equivalents · Greatcoat Grey equivalents