File structure:

gym-env-skeleton/
  README.md
  setup.py
  customized_env/
    __init__.py
    envs/
      __init__.py
      customized_env1.py
      customzied_env2.py

Add any other dependencies for this customized environment in setup.py

If want to add more environments:

1. Add environment files in envs folder
2. Register new environments in customized_env/__init__.py
3. Import new environments in envs/__init__.py

To install customized environment, within project folder, type pip install -e.

To import environment, use from customized_env.envs import Customized_Env

More info can check https://github.com/openai/gym/blob/master/docs/creating-environments.md