Gu Ya arranged quite a number of people to protect Su Yunjin, including Xue Chu and Hong Wu, both of whom were skilled masters. For Prince Chu to abduct Su Yunjin, he must deliberate at length. Directly attempting to raid the Gu Family would surely alert the guards Gu Ya arranged to protect Su Yunjin, inevitably leading to a fierce battle between the two sides.
Gu Ya is close with Prince Ning and is highly regarded by him, along with the Royal Prince and his wife who also have ties with Gu Ya, so directly seizing Su Yunjin is inappropriate.
Open means won't work, then it must be done in secret. Prince Chu's subordinates are not lacking in talents. Although Su Yunjin is protected by people arranged by Gu Ya, abducting her wouldn't be a major difficult task for Prince Chu, if he plans meticulously, he can accomplish this.
