Every GST rate is charged either as CGST + SGST (or UTGST) or as IGST. Which one depends on where the supply is made, not on where the buyer is registered.
Intra-state supply — supplier location and place of supply in the same state: charge CGST and SGST, each at half the rate. In a union territory without a legislature, SGST is replaced by UTGST.
Inter-state supply — place of supply in a different state, and all imports and exports: charge IGST at the full rate.
For goods, the place of supply is usually where the movement of goods ends for delivery. For goods sold over the counter, it is the shop. For most services to a registered business, it is the recipient’s location.
Charging the wrong tax is not fixed by a simple adjustment: you pay the right tax and claim a refund of the wrong one, so getting place of supply right at billing time matters.
IGST credit is used first against IGST liability, then against CGST and SGST. CGST credit is used against CGST, then IGST. SGST credit is used against SGST, then IGST. CGST credit can never pay SGST and vice versa.
Billing, purchases, stock, party ledgers, and GST reports on proper double-entry books. Free, in your browser.