ΔE 19.5, loose: different enough that you should pick on purpose. Grey Spell is a little lighter, more muted, warmer than Nighthaunt Gloom.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Grey Spell is a little lighter, more muted, warmer than Nighthaunt Gloom.
The same difference applies the other way round: Nighthaunt Gloom is a little darker, 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 Nighthaunt Gloom on the colour wheel See Grey Spell on the colour wheel
All matches: Nighthaunt Gloom equivalents · Grey Spell equivalents