ΔE 16.4, loose: different enough that you should pick on purpose. New Gold is much more muted than Gehenna's Gold.
Can I use New Gold instead of Gehenna's Gold?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). New Gold is much more muted than Gehenna's Gold.
Can I use Gehenna's Gold instead of New Gold?
The same difference applies the other way round: Gehenna's Gold is much more saturated.
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.