Blue Horror vs Gigawatt Blue

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Gigawatt Blue is a little darker, much more saturated than Blue Horror.

Can I use Gigawatt Blue instead of Blue Horror?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Gigawatt Blue is a little darker, much more saturated than Blue Horror.

Can I use Blue Horror instead of Gigawatt Blue?

The same difference applies the other way round: Blue Horror is a little lighter, much more muted.

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.