Is your feature request related to a problem? Please describe.
No. This is a development proposal for a lack of a function, which includes the lack of training rooms and gym masters.

Describe the solution you'd like
Gyms and gym masters could be included in the next development, once the elite 4s are in.

Describe alternatives you've considered
There is no alternative solution. Its function, as known in the video game, the gym master gives you a badge and one or a maximum of two emeralds.

Additional context
I can't add anything.

And in which city? Because I've traveled all over the world and haven't found a single one.

worldgen towns already have gyms and gym leaders, the gym leaders give an appropriate badge when defeated

in the ones which are pokecube:town_<biome> in the locate menu, it is not as common as the pokecube:village_<biome> ones, which do not have the gyms.

There is also an elite four structure, but it doesn't really have requirements to challenge atm, but should have fairly high lvl pokemobs