Question: can gohan go beyond mystic?
Answer: Yes, Gohan can transform into SSJ in its mystic form, but it will be useless. Old Kai unlocked the hidden power and strength of Gohan at the time of Majin Buu, due to which he became far more powerful than SSJ1, SSJ2, and SSJ3 Gotenks, and hence turning into a SSJ would be a waste.