Hi,
So your product does not have any tax category or a speical one, to be "tax free", right ?
In order to add the text "gst free", you can do it using view override (checking the product tax and display the message if so).
Reading the "product_tax_id" is possible in most of views (in some cases you might load the full product using the order product object).
You can otherwise, use a product custom field and use this custom field to display the text where you want to display it. But the product_tax_id will be I think, a better solution.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.