The core accretion model is the most widely accepted model of gas giant formation: that a solid core is produced by accretion of solids from the protoplanetary disk up to the order of 10 Earth masses, followed by the accretion of gas (core accretion). The core formation is much like the formation of rocky planets, but if beyond the snow lines, has the advantage that ice particles might contribute to the core growth.
The other model of gas giant formation still under considerable consideration is the gravitational instability model, in which instability within a protoplanetary disk allows portions of it to gather, resulting in a sufficient blob of gas to accrete more. 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.