My gauge reads just below zero when the key is on, but the engine not started. Shouldn't it be at 0?

yes it should

Um, with the engine off...

Sidenote, I can get boost in neutral with a quick stab at the gas.

I'll have to check my ignition on/motor off reading and get back to you, but all of us can get boost in park or neutral for a half-second or two.

It should read zero. The air in the intake is at atmospheric pressure so their should be no vacuum or increase in pressure (aka Da Boost). Probably just the typical analog guage thing where they can be inaccurate from time to time. Plus, the fact that the cluster is at 12.6 volts vs it'* operating voltage of 13.8 might have a minor influence. FWIW mine is the same, reads like .5 of vaccum when key'* on engine'* off.