ΔE 44.3, loose: different enough that you should pick on purpose. Pearl Blue is much more saturated than Gun Metal. They are different kinds of paint: Army Painter Gun Metal is a metallic and Tamiya Pearl Blue is a pearl paint, so they won’t behave the same on the model even where the colour is close.
Can I use Pearl Blue instead of Gun Metal?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 44). Pearl Blue is much more saturated than Gun Metal. They are different kinds of paint.
Can I use Gun Metal instead of Pearl Blue?
The same difference applies the other way round: Gun Metal is 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.