Goods were sent on approval before the appointed day but are returned to the seller after 6 months from the appointed day, will tax be payable in GST?

Yes, if such goods are liable to tax in GST and the
person who rejected or not approved the goods returns it
after 6 months (may be extended by 2 months) from the
appointed day.
This provision is applicable to SGST law only – section
162D