First thing GPA is not valid mode of trasnfer of property. It is not part of document for transfer. Also, GPA can be revoked in future by the original owner of the property. Whether you can sell the property can be checked after going through GPA. However, if it is assume that GPA allows you to sell and if you want to be owner of the property then you can sell that property to yourself.
Is GPA is registered, is GPA accompanied with agreement to sell, valid receipt of payment of consideration? Have you informed the office about purchasing? Have you shown the same as your asset in IT? If answer is yes then after selling the flat you shall according be required to inform your office in this regard.