Is gravity an obstacle for homogenous solvation due to diffusion?
At normal Earth gravity, for all practical purposes homogeneous solvation is not hindered by gravitational force.
However: For some solutes such as sucrose and $\ce{CsCl}$, though, the &quot;artificial large gravity&quot; of centrifugation is capable of establishing concentration gradients in their solutions. One example in common laboratory use is buoyant density centrifugation:

One of the most common methods of fractionating DNA is to exploit differences in density between different types of DNA in concentrated salt solutions. Caesium chloride is the most commonly used. The method involves prolonged high speed centrifugation of purified DNA in concentrated caesium chloride. The centrifugation produces a gradient of salt concentration in the centrifuge tube and the DNA migrates to occupy a position in the gradient which corresponds to its own buoyant density.