Grind the area back like you’re grinding out a crack to fill with weld. Sand back the gel coat like an inch or two around the crack. Soak fiberglass strips in resin and layer it in there. Start skinny and then do enough layers to get the thickness you need to make it a bit proud of the original gel coat, each time going wider and covering more area. Throw some glass on the backside, too. I usually add a strip of aluminum or something to cross the crack on the backside so it adds rigidity. Sand the front side smooth and paint it white to match your mismatched fenders, because prerunner
Another thing people over look is the least amount of resin the better. They actually make special rollers that are supposed to help push out the resin. Like bodj said the more u can v out the Crack almost all the way through the better the patch will be almost like feathering it out as you go. They also make a mesh woven fiberglass that is similar to carbon cloth that works well too. kitty hair is good when u have to fill or build up superficial stuff. It is pretty strong as well.