How to Settle GPA and Freehold properties among brothers

My grandparents didn't write any will while they were alive. They have one freehold house property which is currently getting used by their four sons to live jointly. Also, my grandfather bought a shop for my father in around 1984 where he is working from then till now. Now my father and his 3 brothers want to settle the properties in a way that my father gets the shop but no rights on the house and others 3 have the equals right on the house but no right on the shop. But the problem is that the house is freehold property but the shop is on GPA. And We have heard that settling on GPA is not trivial, my uncles can later deny the GPA settlement and can claim for the shop. May you please suggest how we can approach this matter so that there is no risk in future for someone to legally claim for other's settled property portion? Also, Do we require all grandchildren agreement also or only my grandparent children (i.e my father and uncles) agreement is sufficient?