every gym leader is like “I lost!?! UNBELIEVABLE!” buddy you live in a world where every ten year old child has always been offered a free fire breathing monster at least once and you brought nothing to this fight but anthropomorphic flowers
gym leaders’ whole job is to provide a specific challenge, a battle of a certain type and difficulty level. if you’ve brought the tools and skills to complete that challenge, you’re going to win by design. the pokémon in that battle are probably not actually the strongest pokemon they have.
when gym leaders go “argh, how could i lose??” they’re acting to give your victory legitimacy because you’re 10. they’re like a villain cosplayer letting a baby knock them over. they’re being nice!!