Apparatus and method for passive gas separation in a through-tubing-conveyed (TTC) pump system comprise abruptly reversing a flow direction of wellbore fluids from flowing up the wellbore to down into a liquids reservoir. The generally annular liquids reservoir is formed between a production tubing and a pump of the TTC pump system. Wellbore fluids flowing up the production tubing are diverted into the annulus via one or more outlet ports in the production tubing. The wellbore fluids flow up the annulus until encountering one or more inlet ports in the production tubing, which redirects the wellbore fluids from the annulus down into the liquids reservoir. The reversal of flow direction causes gas in the wellbore fluids to separate from liquids as the gas continues moving up the wellbore instead of changing direction with the liquids. The substantially gas free liquids are then pumped by the pump up to the surface.