Once you have pulled the gaitor and its frame over the top of the original gear knob, try to open out that crimped part of the retaining clip, you can see the retaining clip over the green fitting in one of the pictures posted.
By doing that, I used two smallish screwdrivers against each other - if that makes any sense, to easy open - a bit - that retaining clip, then once you have opened it up enough, pull the original gear knob off.
If you are fitting a SEAT etc different gear knob, you will need to crimp the retainer on the new knob - or if a genuine part, it might also come with a new gaitor fitted.
I did this, to my wife's previous Polo as I wanted to fit a genuine leather gaitor to it instead of the crappy leatherette, the bad news was, VW had already fitted a genuine leather gaitor to that car, it was just very hard and plastic! Separating the old gaitor from the gear knob was a right pain in the bum as it was retained by a plastic ring over a plastic mount, lots of bad words, but I got it done okay!
Edit:- the last thing I'd want to end up having showing in any of my cars is a crappy sharp cut off end of a tie wrap, try to keep that hidden - ie inside like VW group intended!