Indeed one can use the gunwale for leverage on the shaft of the paddle during the J-stroke or the rudder stroke, which is also known as the thumb-up J-stroke, goon stroke or stern pry in white water paddling.
The problem with using the gunwale for prying when doing a correction stroke, is that it can be bad for your stability. Not something you may feel directly in stable touring canoe on flatwater, but in heavy waves or in a tippy solo canoe like the Mad River Canoe Pearl or the
Twister, you can sure notice the bad effect it can have on your stability, especially when you are paddling with a (too) long paddle.
I remember someone from the BCU during the International Canoe Symposium in the Netherlands, who 'suggested' that it would be stupid [sic] not to use the gunwale for prying when doing a correction stroke. However, the next day when trying to paddle the
Twister, he quickly returned with just a few strokes because his prying correction gave him too much stability problems in this canoe. Admittedly, he also used a too long paddle, so that wasn't helping too.
So I won't argue that using the gunwale for prying judiciously can be useful in certain situations, but being able to paddle without it, is more recommendable in my view. Because even in stable boats the negative effect of prying of the gunwale may turn up in critical situations where problems with stability are not appreciated.
Another point is that when one uses a pitch stroke to go straight, the paddle shaft (has to) move a bit away from the gunwale (unless you are a bow paddler) so then it is impossible to use the gunwale.

Dirk Barends