If you can gain access to the rear of the area with the cracked gell coat it is often worth your while to bond an extra patch of fibre glass matt on first. Then do the gel repair.
Gel cracks often appear where the GRP can flex, or often near corners where good fibre glass work should have extra thickness.
I did an old A frame shroud and it soon cracked again. An extra layer of matt on the inside adds little weight but the spidering gell cracks never returned.
Cut the gell crack into a V with a sharp stanley knife and fill with gell repair in stages.
