The transitions are based on the amount of surface gas, it's not picky if it's Co2 or another gas. It's at a point where the gas amount is so high the planet would be visually opaque. Calling it a gas giant is a bit of a misnomer though, it's more of a Venus style scenario at that point.
The transitions are still pretty rough, and we're working on the classifications of planets still. This isn't really a "bug", but it's definitely something we want to improve the handling of as time goes on.