The -7/+7 gauge is in 100 mmHg.
That is, millimetres of mercury.
Yes, I'm serious, and no, it's not commonly used, even in Japan, so I don't know why Nissan chose to use it.
the "+7" -is in units of 100mm Hg (mercury), so the +7 = 700mm Hg, which is equivalent to 13.5psi.