Is it possible to store a JSON file to an ENV variable with dotenv?

Convert the JSON object to string and store it in the ENV

You can use JSON.dump to convert JSON object to string

and then in your controller JSON.parse(ENV['CLIENT_SECRET'])

Alternatively

you can create a google_session.rb inside initializers folder

$google_session = GoogleDrive::Session.from_service_account_key(
   # config goes here
)

and in your controller you will have access to $google_session global variable