Hmm, I was always of the opinion that gym leaders were weak to a specific type of pokemon that could only be found recently, for the most part. At least for a certain point.
Brock - Weak to 2/3 starters, and a butterfree can take care of him pretty efficiently which can be caught in the immediate area. Same with a Nidoran. Yeah, bad typing, but meh. It forces you to learn about typing, as well.
Misty - Immediately north of Cerulean is the first Grass type you can get in the game. Good chance to train it against Misty?
Lt. Surge - You can go out and get Diglett immediately before battling him for sure, even assuming you didn't get Geodude when you had the chance, and somehow hate Sandshrew.
Erika - First fire type can be caught immediately preceding meeting her, ignoring the boatloads of flying types in first gen.
Blaine - Surf. If you wanted to raise a specific water pokemon, here's an excellent chance, especially where every single water pokemon just became available to you.
Koga, Sabrina, and Giovanni - I can't figure any of these guys out, at least not in such a concrete sense.
Psychic had no exploitable weakness in Gen 1, you fought it out the hard way. Interestingly enough, it was put next to a massive, massive dungeon for powerleveling that needed to be cleared before you could fight them, and there was another gym to show just how powerful psychic is. Originally ghosts were meant to be good against Psychic as well, which is next door. Overall meant to force you to use your best pokemon, as opposed to type cruising like most gyms allow.
I can't figure out any sort of explanation in this sense as to poison and ground being where it is. I think at this point in the game, the designers assumed if you were going to use typing, you'd go back and get a ground or psychic type in poison's case, and in ground's case... you just beat blaine. You must have a water.
This might only be half an argument, though. It's possible to subvert this and say that to some extent, pokemon were placed based on who you had to fight next. This isn't as important in later generations and in later points in first gen because, well, you could catch more. You could have all three elements and another powerhouse before you so much as fought the first gym.
Brock - Weak to 2/3 starters, and a butterfree can take care of him pretty efficiently which can be caught in the immediate area. Same with a Nidoran. Yeah, bad typing, but meh. It forces you to learn about typing, as well.
Misty - Immediately north of Cerulean is the first Grass type you can get in the game. Good chance to train it against Misty?
Lt. Surge - You can go out and get Diglett immediately before battling him for sure, even assuming you didn't get Geodude when you had the chance, and somehow hate Sandshrew.
Erika - First fire type can be caught immediately preceding meeting her, ignoring the boatloads of flying types in first gen.
Blaine - Surf. If you wanted to raise a specific water pokemon, here's an excellent chance, especially where every single water pokemon just became available to you.
Koga, Sabrina, and Giovanni - I can't figure any of these guys out, at least not in such a concrete sense.
Psychic had no exploitable weakness in Gen 1, you fought it out the hard way. Interestingly enough, it was put next to a massive, massive dungeon for powerleveling that needed to be cleared before you could fight them, and there was another gym to show just how powerful psychic is. Originally ghosts were meant to be good against Psychic as well, which is next door. Overall meant to force you to use your best pokemon, as opposed to type cruising like most gyms allow.
I can't figure out any sort of explanation in this sense as to poison and ground being where it is. I think at this point in the game, the designers assumed if you were going to use typing, you'd go back and get a ground or psychic type in poison's case, and in ground's case... you just beat blaine. You must have a water.
This might only be half an argument, though. It's possible to subvert this and say that to some extent, pokemon were placed based on who you had to fight next. This isn't as important in later generations and in later points in first gen because, well, you could catch more. You could have all three elements and another powerhouse before you so much as fought the first gym.