But won't be the new (not the gas giants former) atmosphere form from the volcanism of the naked core, producing CO2, SO2... I mean standart volcanic gases, if we assume that the core is composed of what an ordinary terrestrial planet is?Are are gas giant cores supposed to have much different composition?Enlighten me please, I was under the assumption that the core is basically a super terrestrial planet after the former atmosphere gets blown off, and that because of its mass it will have great volcanic activity that will release a lot of greenhouse gases including CO2 and water vapor, or no ?

Will the volcanic activity release just a lot of nitrogen and argon due to the remaining core not containing CO2, water etc.?I would guess that more CO2 than N2 will remain in the core, due to N2 being less dense than CO2 and so N2 being more able to escape from the planet in the evaporation phase of the gas giant.
Argon is more dense than CO2, but I think much less abundant, if our Earth had no limestone, carbonates, dissolved CO2 in the waters etc...its atmosphere would have about the same amount of CO2 as Venus and Earth has retained all its noble gases in the atmosphere expect for helium since its birth but still, only 1 percent of our atmosphere is argon so if there was no carbon cycle there would be about 10 000x more CO2 than Ar.

How much gas could a Cthonian planet contain to release through volcanism?