Answer:
Gravity may cause the density of a medium to vary with depth.
For example because we have gravity the pressure in water is much higher 1 kilometer below the surface than just 1 meter. This is because the water at 1 kilometer down also has to carry the water above it. If there was no gravity water would not fall down, and as such there would be no pressure (and thus no density difference).
Without gravity objects would not sink. Think of it this way; the only thing that separates up from down in water is that gravity pulls things down. If it was not present up and down would be equal, so you expect the object to respect that and just remain where it is.
By the way, liquid only varies very slightly with increased pressure.