Gothic Blue vs Cygnar Blue Base

How close are they?

ΔE 18.8, loose: different enough that you should pick on purpose. Cygnar Blue Base is a little lighter, much more saturated than Gothic Blue.

Can I use Cygnar Blue Base instead of Gothic Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Cygnar Blue Base is a little lighter, much more saturated than Gothic Blue.

Can I use Gothic Blue instead of Cygnar Blue Base?

The same difference applies the other way round: Gothic Blue is a little darker, 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.