In fact, Gu Chengyu was not unwilling to go guard the border, but with his rank, going to the border would never allow him to become the main general.
If he becomes the deputy general, he'd be under someone's control, and his actions would be inevitably restrained; what would be the point then?
Moreover, the Crown Prince was already wary of him and would never allow him to wield military power alone. The Crown Prince understood that he was not the one the Emperor ultimately chose, thus he remained anxious.
If Gu Chengyu obtained military power, and future situations turned unfavorable, he could assist others in overthrowing the throne, ending the Crown Prince's regal days.
Understanding this, the Crown Prince absolutely wouldn't allow Gu Chengyu to hold military power, hence Gu Chengyu had little interest in going to guard the border.
