Just leave it for a day... Was expeeting wind shift from south but not that strong as it turned straight to the bow which was held by only one mooring line. And then the transom had a long french kiss with pontoon. Luckily neighbours noticed "jumped out" fender and did adjust the situation but the damage was already done.
Sanding and sanding after polyester filler with fiber glass.
Adding polyester putty (could have gone without it but I felt I need to experiment) and sanding again.
Applying gelcoat and oversanding.
This was my first serious try and I will admit it was not perfect. And I had to sand a bit more and to apply more coats of gelcoat. Main thing is to keep it waterproof :)