ΔE 36.0, loose: different enough that you should pick on purpose. Overlord Brass is noticeably lighter, more saturated, warmer than Gemstone.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Overlord Brass is noticeably lighter, more saturated, warmer than Gemstone.
The same difference applies the other way round: Gemstone is noticeably darker, more muted, 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 Gemstone on the colour wheel See Overlord Brass on the colour wheel
All matches: Gemstone equivalents · Overlord Brass equivalents