Is it possible that the gas giants are really just unfinished planets?

I was recently watching a special on Nat Geo about Jupiter and they were saying that nobody really knows how the gas giants were formed or why they exist as they are. I started thinking: if all planets are theorized to come from giant disks of gas and dust, and the gas giants happen to be giant of gas, is it possible that they are really just unfinished planets (taking into account that their mive size would require an insanely mive dust cloud, which presumably would take much longer to create a planet)?