Which is best to repair a crack all the way through a Royalex hull prior to armoring it? I have used 3M before and had good results as it is very strong and flexible, the same attributes G-Flex is touted as having.
If you get on the Georgia Canoeing Association site to ask this question there is a person there who is a wealth of knowledge and could readily answer your question. Don't want to give his name for obvious reasons.
Hope this helps. Nitro Vinny