ΔE 15.8, loose: different enough that you should pick on purpose. Pilgrim Grey is noticeably darker than Longbeard Grey.
Can I use Pilgrim Grey instead of Longbeard Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Pilgrim Grey is noticeably darker than Longbeard Grey.
Can I use Longbeard Grey instead of Pilgrim Grey?
The same difference applies the other way round: Longbeard Grey is noticeably lighter.
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.