GST setting can be done at the event level. On the ticket page[Event Setup/Tickets], you will see the Payment setting option on the right side Panel. Click that one and scroll down to the bottom of the page. You will see the option "Tax Settings". Please provide tax details here. If you want the buyer to receive the GST invoices, select the checkbox "Send Tax Invoice To Attendee" and fill-up the form below. Invoice on your behalf will be shared with the buyer every time a transaction happens.