The builder is supposed to make sure that there's enough bonding agent to make a reliable seal at the ends. If I suspected the integrity of the seal, I'd use a calking product, preferable one of the clear silicone versions since these make a very flexible seal.
However, be sure to either ask the manufacturer, or test a dab to see if it damages the gel coat. A good place to test in under the BB where the cable guide will cover any damage.
IMO clear silicone caulk is a must for extreme weather cyclists, I use it to weather seal all the places where water may enter, especially the seatpost slot.
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