Gold Leaf vs Radiant Yellow

How close are they?

ΔE 22.6, loose: different enough that you should pick on purpose. Radiant Yellow is noticeably lighter, cooler than Gold Leaf.

Can I use Radiant Yellow instead of Gold Leaf?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Radiant Yellow is noticeably lighter, cooler than Gold Leaf.

Can I use Gold Leaf instead of Radiant Yellow?

The same difference applies the other way round: Gold Leaf is noticeably darker, warmer.

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.