This code runs.

thingofbeauty.php

<?php

$this->getSelect()->group('main_table.giftvoucher_id')->joinLeft(
    array('history' => $this->getTable('giftvoucher/history')),
    'main_table.giftvoucher_id = history.giftvoucher_id', array(
        'history_amount' => 'amount',
        'history_currency' => 'currency',
        'created_at',
        'extra_content',
        'order_increment_id'
    )
)->where('history.amount > ?', 0 or 'history.action = ?', Company_Module_Model_Actions::ACTIONS_CREATE);