The difficulty is that the gel coat will have changed colour with exposure to uv. Mark just did a small gel coat repair to Sulis, and despite being kept undercover for the last two years, the colour had changed so the patch is noticeable. When we had Grace (our BRE) we took her to a specialist to make good a couple of places. They were able to match the colour perfectly (to my eyes).
So in summary, if you want not to see the repair, take it to someone who specialises. If you’re not so worried, I’m sure the filler kit is fine!