We know as a fact that SSJ < SSJ2 < SSJ3 < SSG < SSB . But what about the mystic form that probably Gohan is going to recover? Is there any official info / evidence that shows how strong it is compared to the other forms?
My take on this is that his "mystic" form is just him unleashing all his power without turning into a SSJ which puts some strain on his body / wastes some energy.
Basically even if he could go SSJ3 there is no need for that because he can bring all of that energy out in this "mystic" form without any side effects. So no matter how strong he becomes with this he does not need to transform (regarding the normal Saiyan transformation).
Now regarding the "god" transformation (if he ever reaches it), I'm not sure. Since we have a new kind of ki involved maybe he can not bring that out without transforming into a SSG but that is even more based on speculation.
In the end, Mystic form > all other normal SSJ forms because it enables him to draw out all energy without any side effects.
SSJ2 < Mystic Form < SSJ3
Mystic Form is Stronger than SSJ2.
Mystic Gohan was More Powerful than SSJ2 Goku or Vegeta during fight with Buu, But Buu after absorbing Gotenks(SSJ3) became stronger than Gohan.
Thus, Mystic form more powerful than SSJ2 but less than SSJ3.
SSJ3 Goku was equally matched with Kid Buu. https://www.youtube.com/watch?v=5aMPHOHX9Bc
Kid Buu was way much powerfull than of its any other form.
Gohan could only overpower Super Buu, But was easily defeated by Gotenks absorbed Super Buu.
Hence SSJ3 > Mystics Form
According to some spoilers appearing in a japanese magazine about the upcoming Dragon Ball Super episodes
Gohan is able to hold his own against Super Saiyan Blue Goku in mystic form
Edit: 13/05/2017. After seeing Dragon Ball Super episode 90th
It's quite questionable if Gohan can hold his own against Super Saiyan Blue
Well SSJ3 Goku Was Slightly injured but was still equal to Fat Buu but Mystic Gohan Overpowered Super Buu which was a little stronger than Fat Buu since its Evil and Fat Buu combined.But lost to Buutenks which had the power of at least two SSJ3's which does kinda make Mystic a little stronger than SSJ3 cuz its not rlly a multiplier it just unleashes the energy he can use with all SSJ forms he has (Including Base) combined,without using a lot of energy and stress and Kid Buu shouldnt have been the strongest form out of all Buu (Gohan Absorbed) should have been since its two SSJ3's Combined with mystic and Fat Buu was all Kai's absorbed so he should have gained there power too but they never returned after kid buu's demise so rlly Kid buu should be around a little less than SSJ3 the only reason he overpowered Goku was because his moves were unpredictable since he attacked without thought so That might have been a reason :)
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Some people only see Mystic form as a transformation. It is not. The thing is the Spiritual Gods like Kami, Yama, Kai, Grand Kai, Supreme Kai, God of Something, God Superintendent, Universal God (Dragon God Zalama) are Ranks that can pass pressure on the lower Rank very easy.
Since Gohan is a Mortal, below Kami rank, his potential can be opened by Namekian Great Elder (Kami rank) and even further by the old Supreme Kai (percentage wise). After training, Gohan of course could have gone to the Namekian great elder again, but that guy is dead. Anyway, Gohan unlocked the SSJ2 Stage. When he was a kid he was pretty strong, but he kinda got weaker in his teenage years. He was weaker than SSJ2 Goku and Vegeta. But still, after Dabura fight, he got a little bit stronger again because of the near even fight. Afterward, he kinda was on the level of the cell games. After training and unlocking his potential on the Planet of Supreme Kai, he basically absorbed the SSJ1 and SSJ2 states which makes him put out even more energy that otherwise would have been wasted and he does not get any strain that is accompanied by the SSJ1 and SSJ2 state. He is basically his base state with the power level of the maximum SSJ2 state. So in a way, he is stronger than Goku SSJ3 because he doesn't waste energy that much and has no strain. It's like the Saiyan God state. Every possible transformation tabs into your potential.
So lets make this simple: SSJ1 < SSJ1-Mystic=SSJ2 < SSJ2-Mystic=SSJ3 < SSJ3-Mystic=SSJ4 < SSJ4Mystic=SSG < SSG-Mystic=SSB < SSB-Mystic etc etc.
The Mystic state is basically the transformed strength into the base form which will in turn be always better or equal to the next transformed state. In Gohan's case, it is stronger than SSJ3 but not as strong against Buu that absorbed so many characters. This mystic state basically is the true path to godhood.