ActiveSupport::Dependencies.autoload_paths << "/path_to_gem_dir/gem_name/lib"

group :development do
  # gem "wirble" COMMENTED!, so I can autoload files!
end

group :production do
  gem "wirble"
end

module MyGem
  ActiveSupport::Dependencies.autoload_paths << __dir__

  def do_something
    MyGem::InnerClass.new.do! # autoloaded class
  end
end