The deceased grandfather's intestate property will devolve equally on all his legal heirs of first class, which is - wife, children and mother;
In your case, there is no mention about your grandfather's mother, so let us assume she is no more, then his wife, i.e., your grand mother is alive, he had two sons, assuming he had no other children (including daughters), the property will devolve equally among all his three legal heirs, this 1 acre of land will be shared by them in equal proportion of 1/3rd each.
Out of her 1/3rd share, your grandmother, who becomes an absolute owner of that particular share, she can dispose it in any manner of her choice to anyone, i.e., by way of gift deed, settlement deed, or by sale. She cannot transact with the share of property that doe not belong to her. She will be restricted to her share in the property alone.
The other two sons, if alive will become absolute owners of the property and if anyone or both are not alive, their share in the property becoming intestate will devolve again on their class I legal heirs, one among them will be your grandmother too.
Thus, the above will make you clear about what and how the property can be dealt with.