Can a GPA be issued for some portion of the land?

Hi I am planning to buy an agriculture land of this there is clear title for 73 Guntas, however in-order to get access to the 73 gunta i have to buy 5 Guntas from the adjacent property so that i get connectivity to 73 Guntas. The owner of the adjacent property has agreed to sell me 5 Guntas out of a survey number which is in total 19 Guntas. However since it takes time for mutation to happen for this 5 gunta out of 19 guntas he is telling me that he will give me a registered GPA for 5 GUntas out of 19 gunta (by mentioning schedule of 5 Gunta) and asking me to execute a sale deed once mutation and sketch is done officially at a later point in time. Now my question is: 1) Is such GPA valid? 2) There are 2 owners of 19 Guntas ( 16 guntas with 1 person ) and ( 3 guntas) with another person this split in mentioned in RTC. I am buying 5 guntas from owner of 16 guntas, so does GPA require signature from the other owner who owns 3 guntas? 3) I am planning to do registered GPA in my father-in-laws name and later execute a sale deed in my name for this 5 gunta. Is this fine?