Hi Kim,
Based on the description, I infer that the current requirement is to setup the case as follow,
The use of 0 operating density does not directly provide the hydrostatic pressure condition to the solver. It is done so that the correct pressure field could be setup at outlet.
In your case please use pressure outlet boundary condition. The necessary hydrostatic head at the outlet could be provided through the expression. I have added the snippet below for the same. Please note that the expression would change if the the current mesh position, dimension or the direction of the gravity force is changed. This expression should be hooked at the outlet BC for gauge pressure.
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