Grind it out a little. Let it dry completely. Put tape around the hole leaving the hole exposed and maybe 3/8 inch surrounding it. For 1 cm I would use epoxy with some chopped glass in it with some structural filler. Then sand and re-gelcoat or use my version of gelcoat- 98 cent gloss white spray paint after you take off the tape to blend it. OR.. use marine-tex. You could use marine tex but 1 cm may be getting up there in size for it. easy to use, easy to sand. Hard to mix right in small amounts though.
btw.. didn't read the link given.. so I apologize if this is redundant.

These things are very person dependent. One guy i know who has built boats would recommend grinding out A LOT of the surrounding glass and foam then re-layering cloth, etc. I think that is excessive unless it is a high stress area.