ΔE 15.6, loose: different enough that you should pick on purpose. IJA Gray is much more saturated than Administratum Grey.
Can I use IJA Gray instead of Administratum Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). IJA Gray is much more saturated than Administratum Grey.
Can I use Administratum Grey instead of IJA Gray?
The same difference applies the other way round: Administratum Grey 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.