You should not need much else depending on what comes in the kit. I just did a few gelcoat repairs for the first time a few weeks ago and it turned out really well. I did not buy a kit, but just a pint of white gelcoat with wax. Make sure the gelcoat has 'wax' in it which helps it cure in the presence of air, otherwise you need to cover it with plastic to keep the air out.
Here are a few other things that came in handy with my repairs:
1. disposable tiny parts brushes to apply the liquid gelcoat
2. acetone as a prep and cleaner
3. paper cups to mix the gelcoat (do not use plastic cups, the gelcoat will melt them) and some wood Popsicle sticks to mix in the color and hardener.
4. wet sand paper - I used 300, 1500, 2000, and 3000 to get it near perfect then just hit it with the buffer and looked like brand new. Be careful sanding (especially with the lower grits) and have patience. A flat sanding block will also help alot.
5. masking tape to mask off the area to be repaired
6. search the internet for videos on gelcoat repair and watch them, I learned a lot this way.
7. Dremmel tool to clean out the nicks and scrapes before you add new gelcoat. You want to smooth and bevel the edges for good adhesion of the new gelcoat. Also, absolutely make sure everything is dry and free from moisture.
This is all I can think of off the top of my head, but literally if you can spackle drywall, then you should be able to do these repairs on your own. It can be tricky to get the correct amount of hardener in there so it gives you 15 min. of working time before it thickens so mix up a test batch (or two) to make sure it stays workable for enough time and hardens good given the ambient temperature. Good luck and feel free to contact me if you have any more specific questions.
--Deven