Gohan just learned of a way to get stronger without them. He can transform, he's just weaker doing it. The power he unlocked from within him is something he can't access while being transformed, probably because a transformation has their limits, that's also why there is a super saiyan 2 and 3 because the super saiyan transformation has limits.
Is there any reason that Gohan couldn't transform to Super Saiyan form after the awakening? 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 initial movies. Gohan confirms that not going Super Saiyan is a choice, as it allows him to embrace the human side of his heritage.
Unlike Mystic Gohan, which allows Gohan to reach 100 of his potential, Ultimate Gohan can grow stronger as Gohan grows. Despite his massive strength, "Dragon Ball" hero Gohan no longer goes Super Saiyan - here's why his Ultimate form blows away his previous power levels. Super Saiyan 3 (SSJ3) was the strongest Saiyan form which only Goku had access to, but Mystic Gohan was possibly even stronger.
In terms of who would win, we have to say Mystic Gohan would be a major favourite. Because it wouldn't have done him anything. The Super Saiyan forms don't just multiply the individual's strength out of thin air.
It releases a portion of their hidden potential based on their base form's strength (Super Saiyan releases a portion equal to fifty times their base, etc). The Ultimate form releases the entirety of Gohan's hidden potential and beyond, so there's nothing there for. While in his Ultimate form, Gohan never goes Super Saiyan in Dragon Ball Z or Dragon Ball Super - and there's a good reason for this.
Gohan's Mystic Power-Up transforms his ki into that of a Kai, allowing him to maintain his appearance without changing when he goes Super Saiyan. The Ultimate form gives Gohan the same power level as Super Saiyan 2 but without much strain on his body. Can Mystic Gohan beat Super Saiyan blue? Mystic Gohan can not only hold his own against Super Saiyan Blue, at that point it's stronger than it.
Mystic Gohan is about the same level of Kefla, which is the fusion of Kale and Caulifla, being Kale able to defeat Aniraza and to fight Goku SSB and Golden Freezer at the same time. The first time we see Gohan transform into a Super Saiyan (if you consider it a full transformation) is in DBZ episode 141 - "Bow to the Prince". After the halfway point, we are taken into the hyperbolical time chamber where Goku and Gohan are training.