Gflex is very good stuff... you MUST use a flame on the plastic (HDPE) for it to stick to it. Just passing the flame over it once slowly at a rate of about 6" per second is adequate.
I wouldn't try to use it as a replacement for welding... welding is cheap and easy and should be the first layer of 'fix' for your crack. A patch of Gflex on the inside would act to reenforce the area and decrease flex. I found a really cool tip on the site I bought my welder-
Pelican Canoe and Kayak Repair
Melting metal mesh into the plastic on the inside acts like rebar in concrete. When done properly, you melt the metal completely into the plastic and lay a thin layer of plastic over it.
Using this welder I cut a HDPE canoe in half and removed a 2' section then welded it back together. I welded from the inside and then outside, melted the metal mesh into the inside seam and then layed a layer of Gflex with a single 2" wide piece of fiberglass tape. Its holding up perfect. I've also used Gflex to rebuild the tip of a damaged paddle, worked very well.
As for thickness, the kits come with silica powder thickener so you can add that to get the consistency you want.