Take your boat with you if you go to Fiberlay.

According to fiberlay, opinions may vary, you dont have to take all the gelcoat off, since gelcoat is the same resin with the addition of a small amount parafin for a good surface cure. Marine stores charge more.
Fiberlay has pints qts and gallons. Get the hardener with the cone tip and get the small plastic shot glasses and measure is by the drop.

they also have all kinds of materials. I dont think you need woven roving for a crack. They have a combination matt and cloth back that is smooth. But im confused if the crack is in the hull or a subfloor. If its the hull, you should turn it over and put a layer on the bottom. Again you dont have to go through the entire gelcoat. Gelcoat is very expensive, but they have a shelf with quarts of gelcoat in various colors that did not match there in house project. Those are 25

Now consider the environment

not outside.
Cold weather requires heat lamps to kick it.
cant be raining
sanding dust all over the garage?
sanding blocks, sand papter, grinders, belt or disk sanders
safety glasses
osha mask 26-29 dollars
latex gloves 7-8 dollars
STINK -use fan and ventilate for 12-24 hours
use wax for final coat, or it will sand gummy
Gelcoat is slick, you dont need it for water proofing, but it looks better.
Time
time
time
expect to spend 100 dollars in materials plus tools.
If you can get someone to do the job for less than 300 bucks let them do it.

If its subfloor, then get opinion for fiberglass putty to block the subfloor from flexing and moving. Hull needs to flex no matter what. Matt will be fine.