The overall concept is simple:
1. contain heat below, from transmission through walls and ceiling.
Seal off these cavities from above. Vapor barrier at inner surface.
2. allow moisture entering insulation, and entering attic via leaks,
means of escape. Simple enough through exterior walls. In attic, by
means of venting attic, like with vents in soffit and ridge- sounds
like you're short the soffit vents, which should admit air over all
insulation. Basic physics will cause airflow to occur. This airflow, if
vents are of adequate size for situation, will keep roof skin from
melting snow.

Powered gable vent is for summer cooling, and shouldn't be necessary if
you effect soffit venting to match the ridge vent.

HTH,
J