I bet I have ground out a hundred cracks that look like these. None of them went into the glass. A few times I have found what looked like glass gracks, but after scrubbing it with ascetone and blowing it off I could see the fibers (with a magnifying glass) were not broken. Gelcoat cracks that formed as a result of structureal failures are seldom straight and look more like ZZ, not ll, like these. A tap-test should show if any delamination has occurred in the area around the cracks.
I'm just say'n,
JJ