I always got the feeling that the role of Gas Giants in GCII was simply to float there in the map looking pretty, which was exactly the thing, they had no uses.
Most Gas Giants host a number of moon's, Jupiter for example has 67 or more, with a combined surface area of an earth size planet. That is a bunch of of territory waiting to be colonized.. Many scientists even ponder the possibility of life evolving on such moon's.
I really hope GC3 gives Gas Giants some purpose other then being just for show. At least add the possibility to harvest gases from the atmosphere...