i wouldnt even say most its about half of all gyms so far type reflect on the city or town.
i personally would like to see a dark gym is has never been done before.
Pewter: Mt.Moon is next route and they have a fossil museum
Cerulean: I'll give you this one but it does have a famous river and peer
Vermillion: Again, no explanation
Celadon: While nothing screams Grass, the entire city is surrounded by many trees, more than most other towns atleast.
Fuschia: I have no idea about this one.
Saffron: Silph co. has high intel people to make products, hence Psychic
Cinnibar: Volcano
Viridian: No idea
Like I said before, Kanto didnt seem to have the rule implemented yet. But remember these town used to be in certain colours on the GB and that links to their respective types Gyms
Violet: It's fairly open-spaced so maybe flying?
Azelea: Forest next door
Goldenrod: just a NORMAL big city
Ecruteak: Myths, Myths, Myths
Cianwood: idk
Olivine: idk
Mahogany: Ice path just ahead
Blackthorn: Dragon cave
Rustboro: Mr stone stone=rock
Dewford: is odd and i have no idea
Mauville: Is actually obvious as new mauvill is next door and its a big gambling town due to the game corner
Lavaridge: again obvious
Petalburg: Normal town, doesnt reflect anything major
Fortree: High in Trees
Mossdeep: Intelligent people working in Space station
Sootopolis: WATER
And I can give reason for all of Sinnoh and Unova.....
I know I went abit mad but I'm showing its waaaay more than half the cities.