Hello,
in order to make appear Gorky in 15th American mission, you have to play mission 14, and not help Joan. You have to choose to ignore her, and just rush the mission (go the the north east, it is very quick).
After that, in the mission 15, after the Powell suicide attack, when you are the commander of the base, Gorky will attack you.

czort wie. Niby porzucenie Joan w 14, ale u mnie pomimo iż zniszczyłem bazę w 14 to i tak się pojawił w 15