First of all, whether the investment in buying a house is a gift or a loan, the gift given to relatives of the bank is generally free, and the loan is obligated to be repaid, even if it is a loan between relatives, unless the lender clearly does not need to repay it. If parents register the house as a gift in grandpa's name, it should be in writing, because a law in China stipulates that the gift should be in writing. If parents only lend grandpa a house by borrowing, the house is registered in his name and the house is his property, and there is a loan relationship between grandpa and his parents in the purchase price.
Secondly, even if parents invest in the purchase, there may be cases where the property right of the house is registered in the name of grandpa, not in the name of the investor's parents. According to China's current laws, whoever registers property rights is the owner of the house; Unless there is evidence to prove that the registration is wrong, if there is any objection to the registration of the house, a separate lawsuit for the ownership of the house shall be filed. In addition, if the real estate license is inconsistent with the housing property rights register, the records in the housing property rights register shall prevail. Of course, if it is a house invested by parents to grandpa, but the property right of the house is still registered in the name of parents, there will be no rights dispute and property dispute.
In order to avoid disputes between grandpa and other children, it is best to make a will, and notarization is the most effective. You can also go through the formalities of transferring the ownership of the house when the old man is alive, so that the house will be owned by the owner registered in the real estate license after the change. It is only a matter of time before the will takes effect after grandpa's death. How to choose depends on the wishes of the parties.