Ancestral property

The transfer of self acquired property of your grandfather by a registered gift deed to your father will make him as an absolute owner with clear and marketable title over the property and it will be termed as his self acquired property only.

Therefore there's no need for you to give your consent or to sign the sale deed in favour of the prospective buyer.

Your paternal uncle is not having any rights over your father's property.

During life time ur father, no one has any rights over the absolute property they can claim but it ll not succeed before court of law

If your grandfather has transferred his self earned property to your father by registered gifted then your father has become absolute owner  of property and no one has right/claim over property. Your father has every right to sell/gift or transfer the property to anyone and not required your consent/signature on sale deed.

You,your son or your father's brother have no right/claim over property. 

Your grandfather's gift of property to your father is deemed to be his self-acquired property, and hence, he can do as he wishes during his lifetime.

A property received through duly executed gift deed is personal property. The donee is the absolute owner and he can, during his lifetme,  dispose the same at his discretion.