At the front side of the gaiter you can sometimes feel some clips underneath the plastic (if the gaiter is not too tight). Try unclipping those first. If not i just got my fingers underneath the front peice of plastic and pulled upwards hard
Now the surround is loose, pull the gaiter over the gearknob and you will see a brass coloured ring with a small lip at the bottom of the knob. You need to remove this - you can use a 3mm flat-headed screwdriver into the lip and slightly prise the brass clip away. Try not to distort it too much however if you may need it at a later date.
Once that is off, you can then pull the knob upwards to free it from the shaft (oo-err!). The gaiter is held on by a small plastic clip at the middle. Unclip this very carefully as it can break easily, by pinching the clip together and un-locating the teeth (youll know what i mean when you see it)
Hope this helps