Gas giants are treated as M class planets

[adi]

I've always liked the big ringed gas giants; the mod i'm playing removed them from planet distribution so you only get some in the home systems.
So i added them back and gave them light population (single digits, maybe for the larger ones in the teens); i intended to terraform those.
To my pleasant surprise you don't have to teraform them - you can not - but their small population is already available (like from M class planets) and is added to the system population; i guess those few people are just living on orbital stations.

A small drawback in the mod i play is that gas giants also include wormholes and the badlands graphics; but i don't mind.

Bottom line:
If you want population on gas giants just edit gas giant population with EU; nothing else is needed. Don't have to bother with the "can not be terraformed" property.