Mass and density. Thats what its all about. Jupiter is such a big planet. It has a great deal of mass.The gravitational pull of all that matter is pulling the gas in with such a force as to we can not duplicate here on Earth. Hydrogen gas and metals under so much pressure that the heat is all most up to fusion levels. If Jupiter was four times bigger than it is ( mass not size ) It would be a star. As it is we can not go near it. its radiating far to much for our frail body's to cope with. Lets just visate her moons. They might be friendly.
As well documented above; If you could just pretend it would be OK. You would fall into the planet until you reached the density that could support your mass equally. You would float there forever. As a gas about the same thickness as water. Hmmm...