The easy call is Jimmy G: Let him walk. Let him go reunite with McDaniels in Vegas or Salah in New York. The emergence of Purdy was his worst case scenario for him. It made him expendable and the Niners aren't going to give him a big contract if they know at least one of their QB can be at least him at a fraction of the cost. Jimmy will be fine at his new location doing what he usually does: Win a lot of games but always leaving you wanting more. He will always wonder what if he connected on that deep pass that would have won SF the Super Bowl. He would have been a made man and a Niner lifer.
The trickier call is the Purdy/Lance dynamic: What Purdy did was amazing for the last pick of the draft, but is he the 2nd coming of Tom Brady?.. or is he SF version of Case Keenum, never to duplicate it again?
Trey still has a high ceiling but 3 years without FB is a lot. Eventually, SF needs to know what they have in him. Plus, there is no way they will get back what they gave up to get him. The last thing they can really do is trade him and then he lives up to expectations elsewhere.
Its tricky but it is one that SF needs to get right. The NFC isnt like the AFC with stacked QBs, tell me the team outside of Philly does the Niners need to worry about? If they get it right, its probably multiple super bowl appearance and maybe a title or two. If they get it wrong, the house gets cleaned.