What share can grandson claim in ancestral property?

That was your grandfather's self acquired property n it's hard to get share from that property. Your share depends on the nature of property I.e. ancestral or self acquired. In your case property was self acquired by your grandfather.

Hello,
The property purchased by your grand father was his personal property , and as per the laws there is no impediment in dealing with ones property in which ever legal manner he/she likes . So your grandfather having sold the property to your aunt was an absolutely warranted act and cannot be challenged.
Sons/daughters cannot make claims on father's property till the time the father is alive , and if he dies intestate only then will the property devolve upon the successive legal heirs as per Hindu Succession Act. If the father has left any Will behind then the property shall be divided as per the provisions of the Will.
Yes , the legal provisions are very much there for seeking partition of ancestral property but that too are subject to many conditions. For further details you may contact me.

You have no right in the property of your grandfather and he had the discretion to deal with it in whatever way he wanted. But if there was any HUF existing at that time than you may claim share in the said property.