The g-flex is great stuff, the problem I see is that you have a large area to glue-up, I am not familiar with the Gluvit and have worked with the 5200. 5200 is great stuff too but is runny and may start setting before you get it all squeezed out of the tubes. It ain’t cheap either and neither is the g-flex but you can mix it up and trowel it on using a grooved trowel. Then you have some time, not much, to get the skin on and clamped into place. Be sure you have everything dry fitted first. The g-flex can be sanded smooth and painted, the 5200 is silicone and it doesn’t do that well. It is sold by West Marine.
One more thing, surface prep is the key to success in any paint or glue application, clean it all good with a detergent to get any grease or oil off then use the phosphoric acid to clean and etch the surfaces, a SS wire wheel or brush will also help etch the surface too.