I know this is an old post but thought I'd add to it as I found it really helpful replacing the canvas on my '98 Westy.
I had trouble checking out on the Gunzl site which is possibly due to Brexit so I sourced my canvas from Brikwerks...
https://www.brickwerks.co.uk/t4-parts/t4-westfalia/roof-canvas-t4-westfalia-late-grey.html
Quality and fit was spot on and it also came with an extra window at the front - this is the canvas for the later model but they do the earlier one as well.
To remove the old canvas, I first removed the rubber seal at the bottom from the outside of the van (be very careful not to snap it!). As others have noted, you do not need to touch any screws etc for the bottom, that rubber seal is all that is keeping it in place. Next I undid the aluminium sections in the top of the roof piece by piece. After removing each segment, I took the canvas out and then replaced the section very loosely.
To refit, I did the top first. As all the aluminium profile was in place but loosely, I was able to tuck the canvas in and then tighten the strips up bit by bit. This keeps the canvas in place but isn't yet fitted as the screws need to go through the canvas. I found it a great way of checking everything was in the right place though. Once happy with the positioning, I then lowered the aluminium profile section by section and then secured each piece properly by screwing through the canvas.
I then replaced the bottom seal, trapping the fabric at the base. I started with the back of the roof before working towards the front. When at the front, I lowered the roof, using some blocks of wood to hold it partially open, to take the slack out of the canvas so I could fit it in the seal. I used a bike tyre lever to help push the rubber seal into place - much easier than trying to use your thumbs and I guess equivalent to the tool referred to earlier in this thread.
Took me about 4 hours on my own with a bit of wind thrown in for good measure. I used an electric driver to save my arms for some of the screws and found a small adjustable screw driver really useful for some of the fiddly screws around the hinges at the back of the roof. Only other useful bit of kit I used was a little clamp - I used this to hold the canvas in place a bit further up from where I was screwing in to stop the wind blowing it everywhere.
Hope that helps anyone else taking it on.
ordered new canvas from gunzl last week all pic and info will come in handy, I will read the post a couple more time before starting :crazy
when removing it so if anyone else attempts it be careful when removing! 
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