1) Can my grand mother gift her share ?
First of all grandfather's property cannot be ancestral property, especially since the same was duly partitioned among his legal heirs after his intestate death, even the nature of ancestral attached to the property extinguished with the said partition. Therefore, after such partition the share of property in the hands of your grandmother becomes her own property and she is legally entitled to transact with the property in any manner she desires or decides. She can very well gift her share of property to anyone.
2) Can i file a partition suite if so will all the 18 acers of land get divided to my grand parent siblings ?
In what way you are concerned about this. If at all there is any claim it can be made by your father alone. You have not stated that your father is living or not hence presuming your father to be alive, you cannot seek partition or any share in the property.
3) Can i challenge my grand mother gift deed as i have few evidence to prove the same can i file a cheating or criminal case ?
Against whom are you going to file a criminal case and in what capacity?,
If you are not directly aggrieved by such act, your case cannot said to be maintainable.
4) As its a clear case of cheating can i ask for polygraph test ?
No is the answer.