Greendomize

I've noticed only on cold days in Georgia that when I initially start my 951, my stock boost gauge reads 1-1.5 bar at idle. It will very slowly climb its way back down to 0.4 bar, which usually takes twice as long as the time that it takes for the engine to reach operating temp. The engine runs completely normal when it does this, even while boosting. The aftermarket boost gauge on the car stays at 0 psi while the engine is cold and idling, and it responds normally when I step on the throttle. Once I've driven the car for a while, the stock boost gauge will read 0.4 bar at idle and will function normally again.

What could be causing this?

951_RS

Your aftermarket boost gauge shouldn't read 0 psi at idle unless it doesn't show vacuum.
I would definitely recheck your routing of vacuum lines to make sure everything is hooked up correctly. I wouldn't worry about the stock gauge though other than make sure it isn't leaking air anywhere along the line.

Greendomize

No, the aftermarket gauge doesn't read absolute intake manifold pressure, only the turbo