We propose a pedagogical and self-contained effective description in which gravitational acceleration arises as a buoyancy force generated by pressure gradients associated with an expansion-driven substrate. The framework does not assume gravity as a fundamental interaction. Instead, matter is modeled as a local saturation of a continuous substrate, characterized by a critical saturation density. Under minimal and purely geometrical assumptions, we show that the resulting buoyancy force reproduces an inverse-square dependence on distance in the weak-field, static limit. The Newtonian gravitational constant emerges as a derived parameter of the substrate rather than a fundamental constant. The scope, assumptions, and limitations of the model are explicitly discussed.