Lets imagine a gas giant called Eurus.
It would have a diameter a bit smaller than saturn, but a gravity weaker than earth (92% earth's gravity). It would also be located 15 000 000 km away from its sun-like star. I want to imagine an airborne ecosystem, meaning that the atmosphere of Eurus would be habitable for a floating biosphere.
The atmosphere would be 70% nitrogen 20% oxygen and 10% other elements (mostly water vapor), the deeper we go down, the more water there is until, due to pressure the atmosphere becomes denser and denser until it becomes a water ocean. The deeper we would go, the more pressure there is, less nitrogen and more water until, as I said, there is only water.
Only, I don't know what there would be under the earth-like atmosphere. I was imagining dense water, then hydrogen and finally a metallic hydrogen core, but is it possible, could this composition for Eurus be possible?
Also, if Eurus has a diameter of 114 700 km, at what height would the earth-like atmosphere be located, at what depth would there be a dense atmosphere-ocean suitable for life?