They are stress cracks in the gel-coat. The only way to repair them is to grind all the cracks out and re-do them. I've done similar problems before on fibreglass car bodies and an invisible finish can be achieved BUT the whole area will need painting to match. As the rest of the panel will be coloured gel-coat (rather than paint) obtaining a 100% perfect match with a painted repair isn't always easy.
You might want to try giving the cracks a good clean with an old toothbrush and strong detergent, getting the dirt out of the cracks will make them less noticeable and a good polish after will give them a bit more protection.