Can Gohan transform to Super Saiyan in Mystic form? In DBZ, we see that the Elder Kai fully awakens the dormant power of Gohan. In this state Gohan can match Buu in his unfused form. But after Buu absorbs Piccolo and Gotenks, why doesn't Gohan go Super?

Is there any reason that Gohan couldn't transform to Super Saiyan form after the awakening? 

Edit 1: Removed the transformation in GT from consideration because DBZ and GT are seperate canons.

Edit 2: Dragon Ball Super has canon continuity to DBZ and I've heard talks of Ultimate Gohan in it. Has more information been provided on Gohan's transformation in DBS? Please Note: Yet to see DBS beyond the intial movies.
I think I found a possible answer, because I had a similar question.

In Gohan article on Dragon Ball Wiki.


  However, amazed at realizing that
  some of his power really was unlocked, Gohan apologizes to Old Kai,
  and resumes the ritual. After some time, the ritual is finally
  completed and Gohan is told to power up like he does for the Super
  Saiyan transformation; however, when he does, his appearance does not
  change, but his power and strength gain an awesome increase.


So it seems when Gohan got "Mystic Gohan", even though he increased his power, his appearance didn't change. So he couldn't transform in any other Saiyan form than the normal form, but he was still getting stronger.

I'm not sure if he is able to change, but the fact is, there would be no difference. He already got all his power in his normal form, without a change.