The Lord of Darkness
Rowlet also resists Fighting while Ground would be SE against Melmetal, Pikachu (immunity outside quick attack), Torracat and Lycanroc. Rowlet was Ash's weakest Alola mon and Gladion Has both Umbreon and Zoroark having type advantage against it.Gladion can use Fire memory to counter bug type, even if Golisopod is water type Torracat was able to bring him down below 50% while Silvally is far bigger threat.
Gladion use Fighting type against Ash probably because he is 100% sure that Ash is going to use Lycanroc too while Rowlet is immune to ground type move and who know if Ash had something else like how he pull out Melmetal which was big surprise.
It was clear plot Armor on Ash's side.