GRP etch primer is horrible
It will make the gelcoat more brittle and prone to cracking. It will help promote osmosis (on a real boat that is!). The softening/etching effect is lost after a while so after several layers of paint over the years you will lose whole patches several layers deep when you peel off masking tape. You will need to apply your paint within a specified time. There is no guarantee there is no mould release wax on the surface and etch primer will not work over wax.
Much better to use 180-240g wet and dry sandpaper with warm soapy water after acetone washing the whole thing inside and out, don't trust that the hull maker has done this for you