Question about planet formation and gas behavior.?

I have a question about planetary formation. Pardon me cause there's not much I remember from High School science cl. From what I understand about gas' natural behavior, it is constantly expanding, not really contracting, and never really creating some kind of continual gravitational draw. So for planets to form, there must be some kind of initial gravitational force to pull the planet's gases together, right? How does that work?