The basic melody hasn't changed in 14 years, but the renditions of it certainly have! Which generation's gym music do you like the best?
Also in the list is the anime theme with the same opening melody, titled "Total War".
And to refresh your memory, here are the tracks themselves:
Gen.3: Ruby/Sapphire/Emerald/Fire Red/Leaf Green
Gen.4: Heart Gold/Soul Silver
Pokémon Anime: Total War
Pokémon Anime: Kanto Gym (Intense)
*update* I found an actual Gym theme - it seems like a rare track, actually.