In the ideal case, the Entity will be a one-to-one self-evident, self-explanatory match with the database table collection.

Unless the column naming is not practical : in that case a Zend\Hydrator\NamingStrategy\NamingStrategyInterface implementation could be used.
Unless the column typing is not practical : in that case a Zend\Hydrator\Strategy\StrategyInterface could be used.

It is used to hydrate the entity on Fetch ¹, but not to extract the entity on persist ²

Could we extract everything through the hydrator in persist ?

1. https://github.com/objective-php/gateway/blob/31746ba/src/AbstractGateway.php#L258

2. mysql-gateway/src/AbstractMySqlGateway.php

   Lines 147 to 158 in 9cb3297

   	$fields = array_diff($entity->getEntityFields(), $colsToRemove);
   	$query->cols($fields);
   	foreach ($fields as $field) {
   	$value = $entity[$field];
   	if ($value instanceof \DateTime) {
   	$query->bindValue($field, $value->format('Y-m-d H:i:s'));
   	} else {
   	$query->bindValue($field, $entity[$field]);
   	}
   	}