Evening all, and a Merry Christmas too!

Hi,
you need to find a gel that is colour matched to the existing if it's not painted otherwise use grey or white i.e. neutral

grind back the loose and cracked gelcoat with a pencil grinder/dremil or course glasspaper
clean and degrease
mix gel
apply liberally to area making sure there is no air trapped under it

then you have 2 choices:

apply some packing tape (non sticky side) or similar or wrap some non perforated release film tightly over the repair then stick it in place, you need to ensure it is fully sealed and air tight, once cured (24hrs or so) pull off the tape and give it quick polish

OR

apply loads more than before so the gel repair is raised above the existing surface, the idea is the finished surface level should be fully covered by the additional gel so it can cure properly anerobically, you will probably have to leave this for a few days so the top surface of the repair has gone beyond tacky so you can sand it. The top 1/2mm or so will never fully cure if exposed to air. Once the top surface isn't tacky you can sand down carefully to match the existing levels.

bit of trial and error to begin with but pretty easy, I've done repairs on boats over the years like this and both methods work. I tend to use both depending on the surface shape

That's some great detail, many thanks.

You should put a wax in styrene additive in the gel coat so that it cures and sands well otherwise it will block up your sandpaper and be more difficult to finish.

To be technically correct gelcoats are specifically meant for using in a mould whereas a gelcoat repair can be the same gelcoat with wax added.

You use a small engraver like a dremmel tool to prepare the damaged bit and then add the gelcoat making sure it is slightly overfilled. A tip is to tease the gel with a small pointed tool to be sure any air bubbles are removed. Once cured sand it back flush and smooth down through some sanding grades to 1000 grade wet and dry, then polish.

Most gelcoats are meant for spraying so they are already thin enough for that but it gets even thinner with added wax. There are also brushing gelcoats which are thicker and easier for this repair method. 

Or............use flowcoat which is mostly supplied for brushing and already has a wax additive in it.

colour matching is the hardest part of gel repairs. even if you knew the exact colour code the surface may have faded a bit or different batches of that colour could differ slightly. if you want it perfect youll probably have to mix your own colour by using different colour gel coats or use clear gel with a few different pigments. polish a section first to match to sothe colour will match after youve polished the repair.