Log Gravity Forms validation errors by adding the following code to your theme's functions.php file

log_validation_failure

// log field validation errors
add_filter( 'gform_validation', 'log_validation_failure', 30 );
function log_validation_failure( $validation_result ) {
    $form = $validation_result['form'];
    foreach ( $form['fields'] as &$field ) {
        if ( $field['failed_validation'] )
            GFCommon::log_error( "form #{$form["id"]}: validate() - failed: {$field["label"]}({$field["id"]} - {$field["type"]}) - message: {$field["validation_message"]}" );
    }
    return $validation_result;
}