I've filled in holes in my 2+2 where it had a tow bar fitted with good results.
Firstly, you have to know that gel coat does not set in air. Jim tested this theory but putting a bob on something and left it to air. After a few days/weeks it was still tacky!
If you have a hole to fill remove any loose material. Gel coat really does not like sticking in little scratches or thin layers I found.
If the hole is all the way through the material get a piece of cling film and place it over the bottom of the hole, stretch it quite tight and tape it up really well - I used gaffer tape for this.
Fill the hole with the gell coat with ever so slightly too much mixture, the surface tension should dome the top slightly, you want it about 1/4 mm proud. Hold a piece of cling film over the repair and pull it tight over. The cling film should press hard agaisnt the gell coat and will make some of it over run the edges. Tape this with gaffer tape as well all around.
Take any left over gel coat and put it on a piece of cling film, tape around it to make a little bad of gel coat. Leave this next to the car. Wait about 2 days then takingyour little bag of gell coat, try pressing it to see if it has gone hard. The longer you can leave it the better - I found about a week was perfect. My idea of the bag is so that you can press and feel the gel coat and if its still tacky you haven't damaged the repair.
Once its set, remove the cling film, depending on how proud it is should dictate which grade of wet and dry to use. I used something like 800 then 1200 grit to get the surface back to the same level. Once it's pretty close use T-Cut (cries from people you shouldn't use it!) to get both sides of the repair back to a smooth level finish. Remove any traces of T-Cut by washing the area throughly, polish with a good car polish then polish with over polish.
This will give you a near perfect result, the only problem is colour difference.