Is denying the existence of sound versions of the Gospel during the time of the Prophet considered an act of disbelief (kufr), and is it permissible to acquire knowledge from previous heavenly scriptures?

The questioner has not denied a matter known by necessity in the religion, nor has she lied about anything in the Sharia. Therefore, there is no room for disbelief in what she mentioned. Furthermore, her words do not indicate that her heart is open to disbelief. It is clear from the question that she is afflicted with obsessive thoughts (waswasa), and she is advised to supplicate, humble herself, and consult a psychiatrist.