.We have four main types of planets: solid, tectonic, liquid and gas. Also all planets have one of four chemical compositions: carbon, iron, silicon and sulfur.
well.. i really like the possibility of 80 different planet types, but for the sake of realism i have to create this thread
i think most frequently occurring element in almost every gas giant is H and He.
Some planets have less then 50% H+He (like Uranus) but most have more then 50% H+He (like Saturn and Jupiter).
A gas giant Silicon or Iron planet would not make sense what so ever.
However, in the upper athmosphere, where the dense and temperature are low enough, you can find lots of different molecules.
just my 2 cents,