Is carbon dioxide gas giants possible? Is it possible that a planet originally full of water, and the solar winds decompose the water and hydrogen escapes out, then the oxygen combines with carbon at crust, just like Venus but the whole planets are mostly carbon dioxide and hence become a gas giant?
While not absolutely impossible this is not a likely situation. The reason is that for a planet to be a "giant", then it will have enough gravity to hold on to its hydrogen and helium. Conversely, if you remove the H an He from a planet, it won't be "giant". There are only two gasses that are common enough in the universe to form a massive gas mantle, and those are Hydrogen and Helium.