ΔE 11.8, fair: the same colour family, but not a straight swap. Gorgon Dark Green is a little lighter, warmer than Majestic Fortress. They are different kinds of paint: Army Painter Majestic Fortress is an opaque base/layer paint and Green Stuff World Gorgon Dark Green is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Gorgon Dark Green is a little lighter, warmer than Majestic Fortress. They are different kinds of paint.
The same difference applies the other way round: Majestic Fortress is a little darker, 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 Majestic Fortress on the colour wheel See Gorgon Dark Green on the colour wheel
All matches: Majestic Fortress equivalents · Gorgon Dark Green equivalents