his document was prepared without the consent of mine or my brother or my mother's knowledge. Can I nullify/cancel this gift deed as these things were done behind my family's back ?
The inhabitant in the property shall not confer the title on you over this property.
If this property was your father's own and absolute property, he can very well transfer the same to anyone of his choice.
He need not take anyone's consent or permission or NOC to dispose his own property.
To pressurise him from not to alienate the property anymore, your mother can file a maintenance case against him and also file an injunction suit restraining him from alienating or encumbering the property any further to create a charge over the property for maintenance payment purpose.
Your mother can file a bigamy case also against your father for his act of marrying another person during the subsistence of her marriage with him.