STEPS
1. Go to "Gateway of Tally >> F11 >> Statutory & Taxation"
2. Set the option "Enable Goods and Services Tax (GST)" as Yes
3. Set the option "Set/alter GST details" as Yes and specify the GST details.
4. If you have a single product / service, you can set the GST rate at the Company level using option "Set/alter GST rate details" as Yes
5. Next, press Ctrl+A to save and close the screen