[simonh]

After the planet has formed, yes. You need something Jupiter mass to already exist as a dense planet to retain those gases that way though. Before that forms the gases disperse too easily so you never get to the Jupiter mass planet stage.

It's only if the mass of the cloud in it's dispersed state is significantly greater than that of Jupiter that it's gravity is strong enough to prevent it's constituents maintaining dispersal due to their own heat.