In WGS GST Module, you can find the folder like templates. You can go to FTP of your WHMCS and go to this location "whmcsroot/templates/your template name/" and then upload these three "invoicepdf.tpl, viewinvoice.tpl, quotepdf.tpl" files with your WHMCS.
Note. Before replace these files, kindly take your existing files backup.
