Re: What Can I Use To Fix This
You can cover it up and make it look ok with marine tex and a little paint. Or fill with a polyester gelcoat repair putty and match the color of the purple gelcoat.
The right way to fix this is actually to separate the two pieces of glass (yes, this may mean removing the top cap or part of it) and examine the glass in the area of impact and behind it, remove any cracked or delaminated glass, repair as needed, re-gelcoat, sand, and rejoin the parts. That's an annoying place to get a ding like that.
You'd know better than we would what caused this and how hard the hit was, so you'd be the best person to judge how likely further damage is.
Erik