Allways wondered why Stardock's factions only grew on paradise earth planets...while species from space should live in different enviroments in my eyes.
After searching the forum, I found quite promising ideas here in Stardock's forum to do a step into that direction.
Got some great support from @Hereward_Hills and now built Methane-Breather Homeworld in cooperation with his Planet Traits Mod.
Additional thanks goes to @Deathwynd for backing with ideas and analysis.
It is the first step, as in current state you could only choose homeworld to be extreme world. Further planned updates may bring full conversation into methane-breather factions able to settle on currently unused GasGiant worlds, while facing restrictions on oxygen worlds and of course the opposite for the oxygen lifeforms. But let's take a look on the current built now.
After installing the mod, every custom faction is able to choose gas worlds as homeworld.
In faction creation you could now choose 'Ogle System' as homeworld, which provides your custom faction a gasworld as your new home in space.

After map creation your people start on a GasPlanet

Your new planet offers a base production. Surface shows planet Jupiter art with some workable continents. Think about them as 'floating' stable areas in current build.

Update 1.1:
- more diversification in methane planet style
- added moon
- added possibility of rings
Download MethanBreatherMod Version 1.2
and place the included mod-folder in your C:\Users\{your name}\Documents\my games\GalCiv3\Mods\
REQUIRED installation is Hereward_Hill's Planet Traits Mod
Recommanded and fully compatible installation is Deathwynd's Planetary Diversity Mod
Compatible installation is Hereward_Hill's Alternative Star Map Backgrounds
Have fun and enjoy your new homeworld option.
Cheers!