Can a gift deed be challenged after 6 years

my grandmom gifted a property to my dad through registered gift deed on 15th march 2010.my grandmother died on 8th July 2010.after her death my uncle(dad s elder bro) & his family members are continuously threatening us by saying dis dat we hve done dis execution(property transfer through gift deed)forcefully as my grandmom didnt inform about this property transfer 2 my uncle.we ppl already done d mutation under municipality as well as paying d tax too fr dat mentioned property since last 6 years.so if my uncle file any kind of challenge against dat deed den wat we need to do in dis case?