I was told that all the shingles must be removed. Then the ridge vent is installed over these opening, and new ridge shingles nailed over the vent, making it virtually invisible. It only does it when rain is blown hard by the wind. Snow sometimes comes in as well. Any vent will leak, especially in wind-blown rain and snow.
Most ridge vents are low in profile and wide enough to resist leaking.
Some have fiberglass to prevent leaks. Others have a cotton candy-type fiberglass filler that is supposed to do the same thing. You could replace yours with one of those, or hang a trough from the ridge so that it is a foot or two under the ridge for its full length. Unless of course it freezes, but if it is that cold you are unlikely to have any leaks. As for installing large gable vents, you can try, but the gable vents are what the ridge vent is designed to eliminate.