You have to add such unrecorded sales and additional tax liability in Table 4 of GSTR9 and pay such tax amount under DRC03 (Under 'services' tab - my applications).
Once you select DRC-03, you have to then select FY17-18 and select Annual return and pay through cash ledger.
If your turnover is more than 2 crores you have to file gstr9c also., give valid reason for the above unreconciled turnover there.