The government notification dated 28th June confirms the following tax rates. You can hard-code them as below.
GST 0% - NIL Tax
GST 0.25% (where CGST 0.125% + SGST 0.125%)
GST 3% (where CGST 1.5% + SGST 1.5%)
GST 5% (where CGST 2.5% + SGST 2.5%)
GST 12% (where CGST 6% + SGST 6%)
GST 18% (where CGST 9% + SGST 9%)
GST 28% (where CGST 14% + SGST 14%)
IGST 0.25%
IGST 3%
IGST 5%
IGST 12%
IGST 18%
IGST 28%