You can either make your own Gym or have the builders make it for you. The gym leaders really get to 'rule' over their own towns. They'll have a say on what goes in it and such.
I would like the gyms to be a set type. But the elite four members can have a little bit of a mix.
@Galactico101 Yes, all gym leaders can make their own gyms if they wish. Yes, any Elite four member slot will be open to others in the future besides the Champion slot.