The Gravitational Instability Model is a model of giant Planet Formation positing that the gathering of gases begins with instabilities in the gas cloud making up a Protoplanetary Disk. If the mass becomes sufficiently irregular or patchy, the more massive parts may have insufficient pressure to counteract Gravity and collapse into planets.
This is the second most widely accepted model, the first model being the Core Accretion Model.
These models of planetary formation fit into the Nebular Hypothesis.
Core Accretion Model