ΔE 16.2, loose: different enough that you should pick on purpose. USAF Medium Gray is a little lighter, more muted than WWI German Mauve.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). USAF Medium Gray is a little lighter, more muted than WWI German Mauve.
The same difference applies the other way round: WWI German Mauve is a little darker, more saturated.
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.
See WWI German Mauve on the colour wheel See USAF Medium Gray on the colour wheel
All matches: WWI German Mauve equivalents · USAF Medium Gray equivalents