The first thing I'd do is try to determine whether what is left is stable. From whatI can see it appears as though the underlying laminate is solid. It also appears as though a considerable amount of fairing has been done in the past (Perhaps when it came out of the mold). A bit of sanding should show completely dry dust, nothing should stick together and nothing should flake off. If it seems solid, fill, fair and paint. Sand enough to provide a "tooth" for the filler. I would use an epoxy fairing compound with no glass fibers in it. That stuff is miserable to fair (it is usually harder than the surrounding gelcoat or filler). instead, lay a little piece of fiberglass cloth and unthickened resin in any deep holes that might need structural reinforcement. For fairing, epoxy with sanding filler mixed to peanut butter consistency works well for me. You can also buy pe-mixed filler.
I can't really see the crazing (cracks) from the picture but old gelcoats are prone to crazing and problems with the bond between the gel and the laminate. (Note that these cracks are in the gelcoat only. Cracks that go through the laminate must be cut out and patched.) Crazing can be easily covered with filler or even paint but the cracks tend to show through the surface after a short time and gradually worsen. The best fix is to remove the gelcoat completely then apply filler and paint but that is extremely time consuming. I find the faster methods of getcoat removal like peelers and media blasting too aggressive for thin car bodies. I compromise by sanding as much as I dare, cutting hollows into the bigger cracks, fill, fair, paint and then hope for the best.
Tom