Guntha is only a unit of measurement of land. You need to check the 7/12 and other documents and do title search for such land before purchasing the same. Once you become owner you can but there are some restrictions in selling agricultural land. You need to execute a Mou with developer in order to decide the same. No if the said land is owned by you then you dont need any permission.
Earlier as per gunthewari Act, Gunthewari is the traditional method used for measuring land in the state and several farmers had sold their plots on this basis. However, as per the Gunthewari Act, the practice of creating small plots of agricultural land in multiple gunthas was illegal and hence carrying out constructions on the gunthewari land was not legally approved. As per the Act, the owner of such a house was also not allowed to sell his property.
Presently you can sell the land as well as construct house on it. Maharashtra govt in 2015 has relaxed the rules for the Same