They left a note that in case the plugin was updated to apply this filter:
$this->options['customerID'] = apply_filters('get_eway_customer_id', $this->options['customerID']);
Line added to \wp-content\plugins\gravityforms-eway\includes\class.GFEwayPlugin.php
in protected function getEwayCredentials()
add_filter('get_eway_customer_id', 'return_eway_id', 5, 1);
function return_eway_id($x){
$post_id = $_GET['post_id'];
$eway_account_no = get_field('eway_account_number', $post_id);
return $eway_account_no;
}