Very important to consider exactly what gelcoat
Intended to give cosmetic appeal to fibreglass mouldings (like your deck), g/c is nothing more than polyester resin thickened with...wait..talc, talcum powder..baby powder.
body filler is, usually, ditto but much thicker. Body filler sucks when compared to my old 'lead wiping' training.
Talc is very absorbent and ABLATIVE as our nether regions know .
So I personally wouldn't thicken it any further.
If you put it on really thick (to hell with the cost), then when it gets a bit shiny/slippery in places you can always roughen it up.
Damn the weight, the cost...sand the old surface roughly, toss it on with a rough roller and it'll be there forever.
FWIW, 50 years ago worked 6 months in the f/glassing shop/ tourist coaches.