I'm surprised the gel oat is so thin. Just expected it to be thicker.
The older I get and the more I learn, I realise how little I actually know about boat construction.
Here's some of my limited experience and forgive me if this is Granny teaching you to suck eggs. Nearly all plastic boats are made out of shredded/mat GRP of differing weights and types impregnated with polyester resin in a process called laying up, although nowadays a lot of boats use Vinylester resins. Polyester has great chemical strength and is laid whilst still wet, but is not as good as Epoxy when it comes to mechanical strength i.e. as a glue . Epoxy resin is water proof and has greater strength to weight ratio and has great mechanical and chemical strength, but is much much more expensive, whereas I believe polyester resins are not waterproof and not as strong but are so much cheaper and therefore the cost to benefit ratio makes them the better choice for boat builders.
A Gel coat is therefore added for embellishment, making the boat smooth and shiny and doesn't contribute to much strength, although it does add weight and helps with water resistance, although I learned a while ago that it is not entirely waterproof. Consequently 1 square metre of gel coat , I think weighs about 1/2 a kg and there are a lot of m2 of it on a 20ft boat. So you see, if it was made thicker, you have just added a lot of weight for not much benefit.