Gpa

We have purchased a plot in delhi two yrs before done registery with court and also make it free hold from dda in our name from the name of alotee, Suddenly after 2 yrs a persone came and tell that the plot is his as he had given the payment to the allotee and done GPA in his Late fathers name, now he filed an FIR and wants to take stay on the plot. What we needs to do. Is the GPA valid? and he can takes stay or take the right on the plot