While moving to GST, Trans-1 Form has to be filed with details of the finished goods in stock, raw materials in hand and taxable services obtained before July 1, 2017. The VAT paid on the purchase bills will be credited back as Input Tax Credit to your account.
Now, when you enter the item details in the item creation page of Zoho Books, you must enter the opening stock value and the purchase price of the item, exclusive of any tax paid.
From July 1, 2017, when you sell any goods or services, you must apply the corresponding GST on the invoice.
Books
Simplify accounting
and GST filing.