We already know, we are even taught at school that planets of the Solar System beyond Mars (Jupiter, Saturn, Uranus and Neptune) are "gas-giants". But what does it mean exactly? Also, can any kind of ground vehicles land on its surface?
to answer the question, gas planets do probably have a solid core, because the pressure is so enormous that the atoms connect into a crystal-like structure.
We know that water for example can have more than 3 material states here.
It is even possible/hypothesized that you get an metal if you compress a gas highly enough. This can explain the extreme magnetic fields of Jupiter metallic hydrogen.
But you can't land on this solid core because the preasure is so enourmous.
No, they don't have. Cores of these gas giants are actually solid, but above them, there's a thick layer of gas, with thousands of kilometres of thickness. Obviously, its concentration is increasing as you approach the core, but it's not solid. Bodies are sinking into it, like into wateror quicksand. Except if they have something with smaller density than the gas (like hydroplan's "foots" with full of air)
Because of it, ground vehicles (like rovers) can't be placed on these planets. However, some kind of gliders would be effective in the atmosphere of these planets.
(This is my first attempt to answer my own question, so I may be wrong. Please fix it if I am.)