Force postal code format

Yes, it is possible via the gform_validation hook and custom code. You would have to be sure your field description explains the correct format. We will be launching all new documentation (and site) next week, and the gform_validation hook will have a better example available for users that want to use it to implement custom validation.

The hook is not in the current documentation.

The hook is the gform_validation hook. Here is an example of it's usage:

<?php
add_filter('gform_validation_YOURFORMID', 'custom_validation');
function custom_validation($validation_result){

// set the form validation to false
$validation_result["is_valid"] = false;
$form = $validation_result["form"];

// specify the first field to be invalid and provide custom validation message
$form["fields"][0]["failed_validation"] = true;
$form["fields"][0]["validation_message"] = "This field is invalid!";

// update the form in the validation result with the form object you modified
$validation_result["form"] = $form;

return $validation_result;

}
?>

You would have to know PHP and hook/filter usage in order to put it to use. That example just sets a field to invalid, you would have to add code above that to check the field value and decide if you wanted to set it to invalid or not.