There sohould be no relationship between gelcoat crazing and leaks. Crazing should be largely superficial, and not have much affect on the glass substrate. If the leaks represent water passing through the glass you have much more than a gelcoat repair. So first clarify the source of your leaks.
For what your facing, you should find a good book on fiberglass repair. You will want to remove the crazed gelcoat and apply a new coart of gelcoat. You may or may not be able to develop a reasonable match to the old. If the match is poor, you will need to paint the entire deck so the repaired areas don't stand out.
You may be facing a big job. Maybe you should fix the leaks and live with the crazing..."Good condition" is certainly a relative term.