The gravitational instability model is a model of gas giant formation: the gathering of gas forming a planet 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, which presumes a solid core (much like a rocky planet, but possibly ice) starts the process with gas accretion afterward. Both models have problems, both explaining many extra-solar planet systems and explaining all the solar system gas planets. These models of planet 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. (and there are models of all these). 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.