Pass extra data to finder auth

To my knowledge, there is no way to do this by passing it into the configuration in 3.1. This might be a good idea submit on the cakephp git hub as a feature request.

There are ways to do it by creating a new authentication object that extends base authenticate and then override _findUser and _query. Something like this:

class GymFormAuthenticate extends BaseAuthenticate
{

 /**
  * Checks the fields to ensure they are supplied.
  *
  * @param \Cake\Network\Request $request The request that contains login information.
  * @param array $fields The fields to be checked.
  * @return bool False if the fields have not been supplied. True if they exist.
  */
 protected function _checkFields(Request $request, array $fields)
 {
     foreach ([$fields['username'], $fields['password'], $fields['gym']] as $field) {
         $value = $request->data($field);
         if (empty($value) || !is_string($value)) {
             return false;
         }
     }
     return true;
 }

 /**
  * Authenticates the identity contained in a request. Will use the `config.userModel`, and `config.fields`
  * to find POST data that is used to find a matching record in the `config.userModel`. Will return false if
  * there is no post data, either username or password is missing, or if the scope conditions have not been met.
  *
  * @param \Cake\Network\Request $request The request that contains login information.
  * @param \Cake\Network\Response $response Unused response object.
  * @return mixed False on login failure.  An array of User data on success.
  */
 public function authenticate(Request $request, Response $response)
 {
     $fields = $this->_config['fields'];
     if (!$this->_checkFields($request, $fields)) {
         return false;
     }
     return $this->_findUser(
         $request->data[$fields['username']],
         $request->data[$fields['password']],
         $request->data[$fields['gym']],
     );
 }

/**
  * Find a user record using the username,password,gym provided.
  *
  * Input passwords will be hashed even when a user doesn't exist. This
  * helps mitigate timing attacks that are attempting to find valid usernames.
  *
  * @param string $username The username/identifier.
  * @param string|null $password The password, if not provided password checking is skipped
  *   and result of find is returned.
  * @return bool|array Either false on failure, or an array of user data.
  */
 protected function _findUser($username, $password = null, $gym = null)
 {
     $result = $this->_query($username, $gym)->first();

     if (empty($result)) {
         return false;
     }

     if ($password !== null) {
         $hasher = $this->passwordHasher();
         $hashedPassword = $result->get($this->_config['fields']['password']);
         if (!$hasher->check($password, $hashedPassword)) {
             return false;
         }

         $this->_needsPasswordRehash = $hasher->needsRehash($hashedPassword);
         $result->unsetProperty($this->_config['fields']['password']);
     }

     return $result->toArray();
 }

/**
  * Get query object for fetching user from database.
  *
  * @param string $username The username/identifier.
  * @return \Cake\ORM\Query
  */
 protected function _query($username, $gym)
 {
     $config = $this->_config;
     $table = TableRegistryget($config['userModel']);

     $options = [
         'conditions' => [$table->aliasField($config['fields']['username']) => $username, 'gym' => $gym]
     ];

     if (!empty($config['scope'])) {
         $options['conditions'] = array_merge($options['conditions'], $config['scope']);
     }
     if (!empty($config['contain'])) {
         $options['contain'] = $config['contain'];
     }

     $query = $table->find($config['finder'], $options);

     return $query;
 }
 }

For more information see this: Creating Custom Authentication Objects