This topic has been discussed in countless threads. There's also a good article in the Hotline archives. Here's one thread...viewtopic.php?f=20&t=36472&hilit=Faster+bottom+jobs
As a general rule, you should replace gelcoat where gelcoat has been worn away and replace fiberglass where fiberglass has been worn away. In other words, if you've worn down into the hulls to the point that the fiberglass is paper thin, you need to build back up with fiberglass, not just cover with gelcoat. Fiberglass is a structural material, gelcoat is not. I would not add fiberglass to the inside of the hull. I would add it to the outside of the hull where it was removed so that you rebuild the hull to its original shape.
Also, trying to judge the fiberglass thickness by shining a light through is really not accurate. Once the gelcoat is worn away, the light will easily pass through the glass since there's no foam core at the bottom of the hull and the resin and glass are both translucent. It may appear to be thinner than it actually is.