A sale is not possible with an unregistered GPA.Therefore the sale can be challenged in the court and the deed of sale cancelled.
You don't need to file a writ petition to get information.You can file a petition under RTI and obtain information or get a reply.
It may be noted that a GPA granted in favour of any person with respect to any immovable property per se does not convey any right or title or create any interest in respect of such immovable property in favour of such person. Immovable property can be legally transferred/conveyed only by way of a deed of conveyance/sale deed which is duly stamped under the provisions of the Stamp Act applicable to the state in which such property is situated, as well as registered under the provisions of the Registration Act, 1908. Of course, a power of attorney can be granted by the party transferring the property (transferor) to the party to whom the property is to be transferred (transferee) for the purposes of granting authority to the transferee to execute the conveyance/sale deed in favour of the transferee, and registration of the conveyance/sale deed on behalf of the transferor. However, such a power of attorney itself would not grant any right, title or interest in the immovable property in favour of the transferee, and a conveyance/sale deed would be required to be executed, stamped and registered for this purpose. It may be noted that in some states, such a power of attorney itself will have to be stamped and registered. The above view takes support from the judgement of the Supreme Court of India pronounced in October 2011 in the matter of Suraj Lamp and Industries Pvt. Ltd vs the state of Haryana and others. In this judgement, the Supreme Court has reiterated that GPA transactions are not “transfers” or “sales” and that such transactions cannot be treated as completed transfers or conveyances; however, nothing prevents affected parties from getting registered deeds of conveyance to complete their title. The Supreme Court has further held that GPA transactions may also be used to obtain specific performance or to defend possession under section 53A of Transfer of Property Act, 1882.