It is a common colloquialism for Christians to ask where the Nathan is to speak to David about his wrongs. It's an understandable collquial expression but I have wondered where Gad the seer was when David took Uriah's wife and had him killed. Gad the seer was described as David's personal seer through 1 Samuel while David was on the run.
Gad is the name of a prophet who appears in 1 Samuel 22:5 instructing David to head to Judah. Gad the seer is described as David's prophet in 2 Samuel during the disastrous census. So "if" Gad is the same person in both books where did he go in the middle? If Gad was David's prophet or seer (David had a few advisors, it seems, one of whom was Ahithophel, who turned on David eventually and joined Absalom's insurrection) what was Gad up to when David took Uriah the Hittite's wife Bathsheba and arranged for Uriah's death?
We know from Gad's role in talking to David during the census that he an important role but Gad is absent.
To piggyback on the colloquial question of where Nathan is for David perhaps we can introduce a new element for reflection, where was Gad the seer? Gad seemed to have played a role in advising David previously but was absent here. Perhaps a Nathan appears not simply because David had sinned but also because the prophetic voice David had relied on previously had also failed in some way. Nathan, to go by Samuel and Kings as a cohesive narrative, seems to show up when Gad may be out of the picture or drops the ball. Not that Nathan's scheming with Bathsheba to place Solomon on the throne isn't itself a little problematic but bear with me. I'm proposing that the need for a Nathan may not just involve a sinful king but also the failure of the king's prophet to confront the king. Maybe Gad was advising the soldiers on the field and didn't know of David's actions? I don't know, I'm just throwing that out as a question about the narrative given that both Gad and Nathan play significant prophetic roles in the reign of David. We've seen from the fitful hand-off Samuel oversaw in setting up Saul and then David as kings that even prophets and judges could mess things up.
Still, fallible as even they could be their role in challenging David was important, whether it was Gad who told David what he'd done wrong or Nathan. This may also be a reminder that no matter how great you think your accountability person/network is the reality is that if David's life in the books of Samuel is any indication, that network can fail you and the corrective challenge may come from another avenue you didn't select, didn't anticipate, didn't want, and that hits you with your guilt in an unexpected way by using your own moral outrage as a way to show you your wrongdoing.