Though I thought it was common knowledge, the only direct effect of leaving the manual gear extension door not fully closed is that you cannot retract your gear at all. As has been mentioned earlier, that door switch opens a port within the gear selector valve that prevents a hydraulic lock when manually lowering the gear!
With that port open, no gear retraction possible!
NWS pressure is supplied by gear down supply pressure, so that must be available on the ground, which it is via the gear being selected down!