Gift deed by my grandmother to his one son

Hi, First of all I would like to mention some details :- 1) There is a 100 yards property in Delhi which was bought by my grandfather in the name of my grandmother. 2) They have in total 8 children : 2 brothers and 6 sisters. 3) My grandfather left home in 1991 and never came back( Property remained in name of my grandma) 4) Now my grandmother expired on 8th September 2016. 5) My uncle(chacha) informed us after 1 month that 2/3rd portion of the plot has been given as a gift deed to him by my grandma( She shifted to my chacha's house just 6 months before death and she was too ill to take any such bold step without presence of my father) and the remaining 1/3rd is unnamed. Now my questions are : 1) Can this gift deed be challenged in court as I feel my chacha has forcibly get it done on his name? 2) Do I have any right on my Grandma's property? 3) Does my father has any right on property as all this was done in his absence? 4) Can the case be turned in our favor as my uncle is a fraud and he can stoop any low to get this property? 5) What will happen to remaining 1/3rd portion of plot as she left no will otherwise?