The last letter he sent was like last goodbyes prisoners get to have before they are executed. Jung knew that Gyeom would still be waiting for him in Joseon that's why he said to go ahead in Qing. It was a pretense of waiting for him but in reality he knows that when he learns that Jung is executed he will just live his life withering away in Joseon without the second chance of living a good life in Qing (where he can eventually forget about him and probably live with a new lover without being persecuted).
I think the hint he gave was "I will always be behind you" which can like translate that he will always be watching over him. He didnt say that I will follow you there.
The last letter he sent was like last goodbyes prisoners get to have before they are executed. Jung knew that Gyeom would still be waiting for him in Joseon that's why he said to go ahead in Qing. It was a pretense of waiting for him but in reality he knows that when he learns that Jung is executed he will just live his life withering away in Joseon without the second chance of living a good life in Qing (where he can eventually forget about him and probably live with a new lover without being persecuted).
I think the hint he gave was "I will always be behind you" which can like translate that he will always be watching over him. He didnt say that I will follow you there.