At the very begining, the game asks you 'what place do you like the most'? And there are 4 options: Beach, city, forest, mountains. Depending on your answer, you start in a different place with 2 different Gyms according to the terrain variety (water/ground in beach, normal/dark in city, grass/bug in forest and ice/ghost in mountains, for example).
The region is not linear and you can go to the other places.
Gym leaders use a different number of Pokemon with increased levels according to the number of badges you have. To access routes with stronger Pokemon, there are 'guards' which ask you for a certain number of badges.

Other option would be adding 17 gyms, one for each type, and you only need 8 badges to challenge the league, but you can get all the others for the Z crystals or other stuff.