Anyone got advice on product to fill chips in gelcoat on fibreglass?

Tinting gelcoat is real hit in miss.. takes a lot of experience to get it right.
 I use the basic Nordic (think brand is) from most outlets, smart marine, burnsco, miter 10 etc.

Repairing cosmetic chipping
 A dremmel or similar tool for the drill, round out the chip to solids gelcoat and glass underneath.

Mix up a small amount of gel coat.. usually 1 teaspoon will do 3 or 4 small cosmetic chips.
Wet the ground out area very well.. rub in, with gelcoat mix, and build up with a split ice block stick.
If on a vertical surface.. where as you add to fill in, take a strip of masking tape and stretch very tight over the lower few mm.. and fill in.
Then another strip tape.. fill in.. till fully covered.

Even on flat areas where will not hang, a strip of tape pulled tight over the surface will mean a lot less finish sanding later.
The gelcoat may not have wax added  which means if exposed to air as cures the top surface will remain tacky, messing up sand papers..tape sorts that issue out.

Once cured  pull the tape off.. you may have a few lower points.. a very light dremmel and re fill, but at the same time nick off any high points you may have.. saves a lot of work later and prevents excessive sanding in the good gelcoat around the area.
 Repeat filling and masking tape.

On my boat , I dont bother to finish back to fully sanded , cut and polished surface.. maybe a light sand to block off with a bit of 800 wet and dry then a bit of 1200..if a bit rough.

If was going to sell , then would finish right off blocking and cut polish the whole boat.

Important Stuff
Get the ratio of gelcoat and hardener as close to correct as can.. very important.
 If starts to go off early, dump it , do not struggle to use it up.
 Do not do mix /fill resins  gelcoats on a cold winters day..

Not really unless doing a repaint.. similar as a painting a car and the paint naturally flows out.

Gelcoat has a gloss finish on boat hulls because the moulds are very highly polished.. If someone touches the mould before spraying up the gelcoat into the mould, the finger print comes out in the finished product..

As in my post above, using the tight stretched masking tape stops sagging and gives a flat surface.. no bumps to sand down.. and it is the final block off with wet and dry sand paper.. only takes a couple minutes for most minor touch up chips..finish with 1200 grit , a quick cut of the area and polish gives the final gloss.