Hi
1) You are legally right in stating that the property is not ancestral (as the property is in your mother's name and she had obtained it by way of gift from her grand father).
2) In Hindu law, any property in the name of a woman is her absolute property and does not constitute ancestral property.
3) It appears that the buyers are wrongly presuming that the property is ancestral property.
4) However under Contracts Act, Evidence Act and Transfer property Act, there is a provision for "Consenting Witness" by which You as a legal heir of your mother can sign the sale deed as consenting witness.
5) Please be informed that Consenting witness is different from witness to the deed.
6) The status of consenting witness in a sale deed is similar to that of Seller and the name is included in the sale deed in addition to seller details as a paragraph and the Consenting witness is perpetually bound by the Sale deed once his signature is affixed in each and every page of sale deed.
7)But only thing is the consenting witness will not be paid any monies by the buyer and hence there will be no tax implications on Consenting Witness.
8) Hence full sale consideration will be paid to your mother and she alone will be liable to pay capital gains tax u/s 54 of Income Tax Act.
9) You can ask the buyer to include your name as consenting witness for which i am sure the buyer will have no legal objection.
Hope this information is useful.