Goods were sent on approval not earlier than six months before the appointed day but are returned to the seller after 6 months from the appointed day, will tax be payable under GST?

Yes, if such goods are liable to tax under GST and the person who has rejected or has not approved the goods, returns it after 6 months (or within the extended period of maximum two months) from the appointed day. In that case tax shall also be payable by the person who has sent the goods on approval basis- section 142(12).