I would like to point out that referring to gravitation as a force is not quite correct. Gravitation is not a force it is a curvature in space-time.
A possible answer to your question is this: the curvature of space-time is different in vacuum than in watter for example because watter has mass and contributes to the overall curvature.
post 4 seems good...no material known "blocks" gravity...and all material has a gravitational effect...so does energy, pressure, momentum density,radiation,etc...
The theoretical carrier of quantized gravity is the graviton...never observed experimentally...gravity might someday be included in an overall theory of quantized gravity...called quantum gravity...not yet complete...