ΔE 27.9, loose: different enough that you should pick on purpose. Hydra Turquoise is noticeably lighter, much more saturated, warmer than Gunmetal Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Hydra Turquoise is noticeably lighter, much more saturated, warmer than Gunmetal Blue.
The same difference applies the other way round: Gunmetal Blue is noticeably darker, much more muted, 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 Gunmetal Blue on the colour wheel See Hydra Turquoise on the colour wheel
All matches: Gunmetal Blue equivalents · Hydra Turquoise equivalents