I've had this on my Mk6 5 door - I heard it sloshing around, and discovered the drain holes were blocked in front passenger door. Unblocked them and the water came out.
You could try experimenting with buckets of water, to confirm the slope issue. It could be to do with the angle of the car - if the car is parked facing up the slope, with a slight camber in to the kerb, then I can imagine the water won't come out of the hole until you open the door.
When you say 'gush' - how much water are you talking about? 500ml? Several seconds - 30 seconds?