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. Both models of planetary formation fit into the Nebular Hypothesis.
Gravitational Instability (GI), the condition of density fluctuations receiving positive feedback from gravity, is also considered a means by which the structures of the universe, such as Large Scale Structures, Galaxy Clusters, Dark Matter Halos, etc. In disks, it can result in some specific Turbulence patterns (Gravitoturbulence), such as Spiral Density Waves. It can lead to fragmentation of portions of the disk, potentially the seeds of planets.