ΔE 42.0, loose: different enough that you should pick on purpose. Nebula Copper is noticeably lighter, much more saturated, warmer than Gemstone Red.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 42). Nebula Copper is noticeably lighter, much more saturated, warmer than Gemstone Red.
The same difference applies the other way round: Gemstone Red is noticeably darker, much 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 Red on the colour wheel See Nebula Copper on the colour wheel
All matches: Gemstone Red equivalents · Nebula Copper equivalents