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I'd be a steel gym (favourite type.) The gym would be in an abandoned steelworks that has been reclaimed by rust and overgrown plants. It is within what looks like it used to be an old industrial centre. There are no nearby populated settlements and no gym trainers, just the gym leader alone with their Pokémon.
The music would either be quite ominous or completely silent, accompanied by the ambience of old machinery creaking/groaning overhead, or the occasional roosting bird Pokémon flying off as the player passes them.
There would also probably be some conflict within the story about the gym leader being notoriously difficult to find/never being at the gym when challengers arrive. After battling them, the player can befriend the gym leader and they will appear from time to time to aid them on their adventure.
These guys are some of my favourite Pokémon, too.