How do you take of a gear knob of a vauxhall vectra 06?

It's a press-fit onto the lever. Generally it takes so much force to pull/twist it off the lever that the mechanism lower down holding the lever in place will break before you pull/twist hard enough to remove the knob.

To remove it to replace it with a different knob, use a hack-saw, and cut it in half, taking care not to cut the lever or other items.