To disassemble the gas lift and the base, please follow the two instructions below.
It is okay to damage the gas lift when you disassemble it.
Please look at the first attachment and prepare a rubber hammer.
By hitting the gas lift outwards, you can remove it from the body part of the chair.
Hit in the direction where the green arrow shows, and you will see the gas lift wobbling from the seat base.
The base could be also fallen from the gas lift while doing so.
Please hit the gas lift very hard to disassemble it.
* If you have a pipe wrench, you can use it to shake the gas lift out from the chair top part.
You should hit it with a mallet (a rubber hammer) and will see that at some point the gas lift wobbling.
That is when you can remove the starbase from the gas lift.
* You may also try to hit the top of the base to see if it helps.