"I have a 2016 GT Grade alloy, which uses full-length cable housings."
Where does the front derailleur housing terminate? That is something we can't see from the picture. Good stainless steel cable won't rust, but if water penetrates into the housing, the wire in the housing can rust and jam the inner cable. What sort of cable guide is under the bottom bracket? What we can't see from your picture is just as important as what we can see
If I were a GT dealer, I would make sure to stock up on that part. If yours looks like that on a 2 year old bike, there will be lots of problems for owners in the future