To fill gelcoat cracks open the cracks with a Dremel, mix up new gelcoat, and apply. It's not that hard to do and Boatworks Today on youtube can walk you through the process.
Gelcoat cracks however have nothing to do with structural integrity or leaks. It is simply a topcoat. It looks like you started a 2nd thread so please see my comments there regarding a repair. Flexseal is not a repair and neither is 5200, 4200, or any other sealant or adhesive. They are simply bandaids over a problem that will eventually get worse. Best to fix it nice than fix it twice...