I’m no expert in repairs, but I used to make GRP canoes. The gel coat is there to protect the GRP from damage, including, over time, waterlogging. Fine cracks in the gel coat will not let in much water and need only be repaired for aesthetic purposes. Cracks wide enough to expose strands of glass should be repaired. Bubbles may be a sign of osmosis or internal water coming out of the structure. Whole books have been written about it, but for a dinghy, it’s not the end of the world. (Yachts may be different – they tend to spend a lot more time immersed).
If I had wide cracks or excessive bubbling on my GRP Wayfarer, I’d sand down to the GRP and then build up with stuff called “flow coat”. It is like gel coat, but it dries completely – gel coat always has a sticky feel to it when it hardens exposed to air. My GRP boat actually has some impact cracks about the size of a spider’s web, but they are so fine that I’ve left them alone.