Q:

Why does 2 Samuel 24 seem to imply that Gad was David's only seer?  Or am I misreading the text? 

From 2 Samuel 24:11-12:

11 Before David got up the next morning, the word of the Lord had come to Gad the prophet, David’s seer: 12 “Go and tell David, ‘This is what the Lord says: I am giving you three options.  Choose one of them for me to carry out against you.’” 

Does the grammar of this text imply that Gad was David's only (or, at least, primary) seer, or am I reading too much into this? ( The way that this is phrased in English seems to suggest so, but I'm not sure if this is characteristic of Hebrew too).  If so, what about Nathan the Prophet - what was he? 
Nathan the Prophet is clearly still alive and working for David in some capacity at this point, because 1 Kings 1:8 says "But Zadok the priest, Benaiah son of Jehoiada, Nathan the prophet, Shimei and Rei and David’s special guard did not join Adonijah." 
Was Gad David's primary seer at this point?  If so, what exactly happened to Nathan the Prophet (who was apparently still alive at the time)? 

A:

I cannot see anything here that would make Gad David's only spiritual counsellor.   Indeed, we also know that Nathan the prophet also gave difficult advice to David. 
The construction here is similar to describing one of my shirts as "my shirt" but everyone knows that I will have several shirts.   Or perhaps, describing my local dentist as "my dentist" knowing that the dentist serves many other people as well. 
Therefore the description of Gad the Prophet as "David's seer" would not necessarily make the arrangement exclusive for either party.