“Is Gaelic Warrior the first juvenile hurdler to win the Gold Cup? Commanche Court went close, but can’t recall any others.”
According to wiki, Golden Miller ran in a hurdle at Southwell as a 3yo. The glamour 
A Plus Tard ran over hurdles in France as a juvenile and won a listed handicap there.
Don’t know if The Fellow ran over hurdles before his chasing career or not. He won an AQPS flat race to start with and ran in the Grand Steep at 5, but I don’t know if he had any runs over hurdles at 3/4 in between. I think quite a lot of French horses went straight ove fences (in the years before they ran them over hurdles to sell them to Mullins).
I see Galopin des Champs had one run over hurdles in spring as a 4 year old, but that was in May, same as Long Run.