Does a general power of attorney from a husband to his wife concerning his property constitute a valid gift that would prevent the husband's parents from claiming their share of the inheritance from this property after his death?

What your husband left behind in terms of real estate and other assets is an inheritance that must be divided among all heirs, even if it was registered in your name, because the registration was for tax purposes and not a gift. A gift is not binding until possession is taken, and if the donor dies before possession is taken, the gift becomes void. The mere registration of the property in your name does not constitute a gift, nor does it give you any right to these properties other than your legal inheritance. You must enable the heirs to divide the inheritance as Allah Almighty has commanded. You are entitled to one-eighth if he had children, and one-fourth if he had no children. Beware of wronging the heirs and taking unlawful money, for injustice will be darkness on the Day of Resurrection.