ΔE 17.6, loose: different enough that you should pick on purpose. Hexed Lichen is a little lighter, more muted, warmer than Phthalo Blue GS.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Hexed Lichen is a little lighter, more muted, warmer than Phthalo Blue GS.
The same difference applies the other way round: Phthalo Blue GS 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 Phthalo Blue GS on the colour wheel See Hexed Lichen on the colour wheel
All matches: Phthalo Blue GS equivalents · Hexed Lichen equivalents