Where does Jupiter's gravitational force come from? Why don't Jupiters gasses fly away?
Gravitational force is the attractive force between any two masses.
What about gaseous planets, where do they get their attractive force from?
While the big rocky planets don't have enough attractive force to keep enough gases in their gravitational field, Jupiter has lots of gases.
Does this means there is a concentrated mass inside, under which gravitational field the gases are kept 'in'? And if so, then why are gaseous planets not also considered to be rocky ones?
Gas has mass too. There's no difference between the gravitational force of an atom bound in a rocky planet, and that of an atom freely floating in a gas.
if they don't have any solid mass inside them is an incorrect conclusion.
Some have 'ordinary' solid cores, and the pressures in the centers of gas giants are so high, that they are even 'suspected' of compressing hydrogen into a 'metallic' state. Quoting from the Planetary core Wikipedia page:
Gas giants also have cores, though the composition of these are still a matter of debate and range in possible composition from traditional stony/iron, to ice or to fluid metallic hydrogen.
For Jupiter especially:
Jupiter has an observed magnetic field generated within its core, indicating some metallic substance is present. [... ]
Jupiter has a rock and/or ice core ten-thirty times the mass of the Earth [...]
Thermal contraction/evolution models support the presence of metallic hydrogen within the core in large abundances (greater than Saturn).
The earth's core has an average density of about 12,000 kg per cubic meter, Jupiter's core density is estimated at 25,000 kg per cubic meter (link). Together with its much larger radius than the earth (70000 km versus 6370 km), you will have a pretty massive core with a mass of from 12 to 45 times the Earth's mass, so there's plenty of gravitational pull there.
More to read: The outer planets: Giant planets: Interiors