Well, first you have to remove enough of that badly cracked gel coat to see just how extensive the damage to the fiberglass structural fabric is. To get a patch to bond will require removing the green gel coat down to where you can clearly see fibers. I can’t tell from the photos what you have underneath there. Might be fiberglass roving of maybe just chopper gun sprayed fiberglass strand and resin.
After you determine the extent of the damage you will need to clean the hull up and lay on a patch of at least one layer of 6 ounce/square yard fiberglass and maybe two. You probably don’t know what kind of resin was used to build this boat. If you don’t you will need to use epoxy. If the boat was made of epoxy (unlikely) or vinyl-ester resin (more likely) and you use polyester resin it may not bond. After patching just paint over the repair with green paint and call it a day.