Welcome to the world of stress cracks.
To stop them from spreading you will need to drill a small hole through the gel coat at the end of each spider. The hole will stop the spider from crawling farther. It will not fix the issue which was created from the transfer of load on too small of an area.
To repair them properly you would need to grind out the spider until the gel has a good bond to the glass all around. Then either fill with glass then gel over or you might be lucky and the glass is not compromised and you will only need to re gel.
This is a great video on stress crack repairs