Following methods of the Config.php(app/code/core/Mage/Tax/Model) need to be changed as follows to display the product price excluding GST

public function getPriceDisplayType($store = null)
{
    $groupId = Mage::getSingleton('customer/session')->getCustomerGroupId();
    if (isset($groupId) && ($groupId == 2) &&
        ((int)$this->_getStoreConfig(self::CONFIG_XML_PATH_PRICE_DISPLAY_TYPE, $store)) == self::DISPLAY_TYPE_INCLUDING_TAX){
        return self::DISPLAY_TYPE_EXCLUDING_TAX;
    }

   ...

public function getCategoryPath($sku)
    {
        $product = Mage::getModel('catalog/product')->loadByAttribute('sku',$sku);
        
        $pathArray = array();
        $collection1 = $product->getCategoryCollection()
            ->setStoreId(Mage::app()->getStore()->getId())
            ->addAttributeToSelect('path')
            ->addAttributeToSelect('is_active');
            

        foreach($collection1 as $cat1){            
            $pathIds = explode('/', $cat1->getPath()); ...