Yeah it is stuck pretty well - what I do is undo the four screws from the base of the chair so you end up with the wheels, gas stut and raise/tilt mechenisum - cos the top of the gas strut is stuck in well too. (basically you remove the chair from the base).
Then turn the wheel 'carousel' upside down and place on the edge of a wooded support (I have a small wooden cupboard I'm not bothered about) - You now see the bottom of the gas stut which you hit with a hammer (can use a wooden block so as not to mark it) or as I do just hit hard around the edge (missing the center of the strut) - this easily frees the strut from the wheel 'carousel' - wrap the aluminium around the bottom of the gas strut (should cover the stut with the same depth as the hole in the wheel base) - make sure it's a tight fit into the wheel 'carousel' hole before you 'glue' it in place (allow glue to set) - and reassemble - job done
Note, by glueing the 'packing' to the gas strut, you don't mess up the chair - and the gas strut can then easily be replaced (parts available all over the net) if ever the need.