Oxidized gelcoat will appear porous and chalky (“powdery”).
Minor chalking can be repaired with any number of commercially available Firbreglass/Gelcoat “Restorer” products.
More serious oxidation will require surface abrasion (wetsanding /w 800 then 600 grit), and re-painting (“Awlgrip”, etc).
A re-painting job might run between $200 - to as much as $500 per foot (loa)
BOB'S HOT BOATING
TIPS < Bob's Hot Boating Tips
> has a short, but informative, tutorial on “Maintaining The Quality Appearance Of Your Boat”
Hot Tip Cleaners
Prior to applying any commercial
“Restorer” product - remember that Pre-Cleaning is critical: any contaminants left on the surface will be sealed in after re-coating.