You have not posted what is the personl faith of X e.g Hindu, and the property is self acquired/HUF etc? You have also not posted if there was any condition attached by society ( for poor people) with further sale of property?
Believing that X was HIndu, property was self acquired and ther was no condition attached with further sale of property:
After death of X (without leaving a valid WILL) the ClassI legal heirs of X i.e. Mother, wife, sons, daughters shall have equal share.
If neither of Mother, wife, other sons, daughters were alive then Y is the sole ClassI legal hier and entire proeprty shall devolve upon him, and should acquire the status of self acquired.
Y was to submit death certificate, legal heir certificate etc to concerned authority for recording inheritance. Once name of Y is mutated in records Y acquires status equal to that of owner.
Thereafter Y can sign a vaild GPA to Z.
If GPA is registered and valid the GPA holder :Z, as per T&C in GPA ; dipsose the property e.g; by sale.
Mere payment of property tax may not neccessarily confer ownership, until or unless court of jurisdiction has passed a decree say on valid grounds of 'Adverse Possession'.
You may preferably show all docs on record to an able counsel.
