Gravitropism would always need a physical movement based on gravity which could be sensed by a cell and lead to changed growth. This could take the form of the settling of the protoplast against the cell wall on the lower side of a cell.
Work Step by Step
The key is to see that gravitropism needs a direct sensing of gravity, just as light responses must involve absorption of light. Pick anything in or about a cell which would move in response to gravity, and build you model around that. Settling of the nucleus against the cytoskeleton could also be a basis for a model.