There shouldnt be any gap on top of the garage door if you install a threshold kit, it would probably move the door maybe .5 inches off the current spot. If the top section of the door has a little too much arc in it, or gap from the outside, there is an adjustment for that on the inside of the door at the very top of each side. But if you have a good seal at the bottom of your door, you dont need a threshold kit, unless water is coming inside from a sloped driveway or something. If your door leaves a gap at the bottom, your door spring tension is too high which makes the door too light.
Adjustment plate looks like this:
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