I understand the process by which the sun will become a red giant and the fate of the inner planets. But what of the fate of the gas giants?
I understand that at the red giant stage the sun will be much larger than it is now with an increase in the solar wind. What effect will this have on Jupiter for example?

Oddly enough, in several billion years, Jupiter may become habitable and suitable for life..with water and oxygen.. as it warms from the expanding sun as earth itself becomes fried. Scientists are looking at ways to move earth into Jupiter's orbit in order to escape its fiery death, but 2 or 3 billion years from now is a long time, and it is unlikely that life will still be in its same form at that time, if it's still around. Eventually, however, the sun will contract and cool and give off no heat or light at all, dooming all the planets that are left to an endless cold and dark orbit around a once majestic star.

Assuming that Red giant is Sol's fate. It may not be, due to mass and type of star it is, mind nobody has watched a sol-type star evolve into red dwarf - we assume from what we know that this is the future based on what we know of nuclear physics and what we assume we know about our sun and stars in general - we may be missing a couple of salient facts such as what if we are not witnessing fusion but, as another theory suggests, stars are products of higher dimensional energies bleeding over into our 3D universe - or maybe our maths are wrong, most of what we know of fusion is based upon our maths - since we are not able to sustain a fusion reaction, there is reason to believe that we may not know the whole story.

Being composed of lighter gasses, and having unknown cores, the addition of heat may have large affects on the outer gas giants. Then again the distance from a Red Giant compared to the amount of heat released may not be very much different for the gas giants today so nothing happens - all is possible.

Assuming that there are rocky metal cores at the center of the Gas giants and that most of the mass is from the atmospheres of compressed gasses, we can assume that the atmospheres would burn off as the excited gasses gain escape velocity.

I'll focus on Jupiter here:

Assuming that the mass is caused mostly by the atmosphere, the mass of Jupiter, therefore its gravity would decrease. Less gravity means a lower escape velocity, which means more gas will escape into space.

There will be a point where the gravity will decrease (through loss of mass) to were Jupiter's moons will escape, assuming for the moment that Jupiter's atmosphere hasn't expanding slowing the orbits of the moons causing their orbits to decay to the point where they fall inward.

If the theories of Jupiter's atmosphere are correct, that the surface is gaseous and that liquid and even solid states of the material are at greater depths making up the Jovian atmosphere (in simplified terms, hydrogen and helium) the liquid and solid states being a product of pressure, we can assume that these layers will convert to gas, burning off as they achieve escape velocity. This may lead to a series of expansions and contractions of Jupiter as it atmosphere is slowly eroded away.

If Jupiter has a solid core of metal and minerals, sooner or later that is what would be left.

Assuming that enough heavier elements, such as oxygen and carbon monoxide and carbon dioxide exist, there may be a rocky planet with an atmosphere. Depending on the size and the mass of the solid core material (lets assume Nickle Iron) would decide how much atmosphere is left behind. There would come a state of balance where rocky/metallic material would have a gravity strong enough to hold on to X amount of gasses depending on their composition, and the amount of heat reaching the planet.

If, on the other hand (which isn't likely since we know that at least Shoemaker Levy is in Jupiter) That Jupiter is all hydrogen/Helium, it would continue to evaporate until the mass, therefore the gravity, is too week to hold these gasses together, thus Jupiter would evaporate to oblivion.

All conjecture, considering we do not know if the Sun can go Red Giant (it is possible that we are wrong and instead of smaller stars going red giant they just burn out collapsing inward to a cinder) and we do not know the compositions of the Gas giants.

David