CryoGaian worlds are Gaian worlds which are mostly covered in ice. Land life is non-existent, but there is some life at the bottom of the deep oceans, and possibly frozen in the ice.
Though Necro Gaian worlds can also be heavily frozen like this, the distinction is that a CryoGaian world still has noticeable amounts of life.
They sometimes occur after the EoGaian stage. The oxygen crisis and removal of greenhouse gases from the atmosphere causes a sudden drop in temperature, which causes massive glaciation to occur. There is a build up of gases over time, which allows the temperature to increase.
Atmosphere will be mostly nitrogen, often with a mix of oxygen. As long as the seas don't completely freeze, oxygen generated by life in the deep oceans will be able to restock the atmosphere. Oxygen levels are likely to be low, but breathable.