He was cautious by nature, and although the intelligence he had gathered indicated that the Xuanwei Sect was on its last legs, who really knew the actual situation? It wasn't impossible that it was a ruse designed to deceive the imperial court.
The saying "a lean camel is bigger than a horse" wasn't without merit.
Therefore, Gu Shanhai's plan was simple: to infiltrate and scout out the situation first. If the Sect was really about to fall, he'd help them on their way; but if it was a sham, he'd take his time to devise a plan.
Little White quickly descended; the area was riddled with radiation, making it clear that the Xuanwei Sect had also been affected by the nuclear bomb during the great war.
However, the Xuanwei Sect's gates were still standing tall, indicating that they must have had some interception and defensive measures in place, which allowed them to survive. Cleaning up the pollution on the outskirts, though, would be no easy task.