Majestic Fortress vs Gnarls Green

How close are they?

ΔE 19.3, loose: different enough that you should pick on purpose. Gnarls Green is noticeably darker, warmer than Majestic Fortress.

Can I use Gnarls Green instead of Majestic Fortress?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Gnarls Green is noticeably darker, warmer than Majestic Fortress.

Can I use Majestic Fortress instead of Gnarls Green?

The same difference applies the other way round: Majestic Fortress is noticeably lighter, 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.