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I have the feeling the answer to this will come down to ''impossible'' but let's ask anyway to be sure.

We know gas Giants have no solid surface and if you tried to land on one you would fall into the planet till you reach the solid core (at wich point you would have most likely been crushed into oblivion by pressure)

But i know gas can freeze if it gets cold enough (even thought i heard something about gas under right conditions being able to freeze/solidify so fast it skips the ''liquid'' state entirely and goes straight from gas to solid)

Now suppose you have a gas giant orbiting at a very extreme distance from it's parent star so it would recieve nearly no warmth from it, or go even further and make it a rogue planet without a star.
Could the ''surface'' (for lack of a better word) of such a hypothetical gas giant become cold enough for it to freeze/solidify into a solid surface one could stand on? (preferrably while the lower layers of the planet under the frozen layer remain gaseous)

Below this paragraph i will add a picture showing on the left a cross section of a ''typical'' gas giant (using Jupiter as a basis) and on the right my hypothetical ''frozen'' gas giant.

The gas-to-solid transition is called deposition it is entirely possible for this to occur when conditions are right, on Earth it causes both normal ground frosts and the more spectacular Hoar Frost. But it won't have the effect you're looking for, under the pressures exerted by a gas giant ice, all ices, are liquids, much as the weight of ice moves a glacier it will cause the "crust" you wish to form to flow into the core of the world. That's one problem, the other issue is that at the very low atmospheric pressure represented by hard vacuum the vapourisation temperature of Hydrogen and other light gases is close to absolute zero so a very deep atmosphere of these gases is always going to form around planets large enough to retain them.

I think it is impossible, for a simple reason: solids or liquid are denser than gases, therefore they would sink under gravity as soon as they would form, similarly to what rain, hail and snow do in our atmosphere.

Even in the extremely remote circumstance that a solid/liquid shell should instantaneously form all around the planet above the gaseous layer, such a configuration would be unstable and would quickly crumble down.

Sure, you can do that, though a planet of typical gas giant composition (hydrogen and helioum, mostly) is not a very good choice because frozen H₂ won't work as a surface. (a) It needs to be too cold to solidify and (b) the solid H₂ will sink.

Far better is a planet that is primarily a mixture of water, ammonia, and similar compounds of common light elements. (There needn't be any rocks.)

The big problem remains cooling the thing off to the point where it is in radiative equilibrium with a surface at perhaps 200K. This will take quite a lot of time, since the internal temperature will be very high after formation -- remember that we can see extrasolar gas giants in the IR. But given time, it will cool.

The trick will then be to get enough of the stuff that can freeze solid to the surface.

It's worth noting that the quintessential hard SF writer, Hal Clement, wrote a novel, Star Light set on such a planet.

So, it's a gas giant, but a fairly odd one with an interior of material that would be gaseous except that it's above its triple point.