It seems like all the info about the Devil Gene is in flux right now…so I really don’t konw currently where they are going.
Back when I was really into the Tekken story (like Tekken 3/4), it seemed as if the DG was a gene that started with Kazuya, and was passed on to Jin through DNA, and as such it added an incredible power through some connection with a supernatural force…that doesn’t seems to be the case any more…at least not fully.
In Tekken 5, Jinpachi, Heihachi’s dad, was revealed, and it was shown that he too had an incredible power within him. It was hinted / straight up revealed that “that power,” the DG, skipped a generation over Heihachi and went to Kazuya and then Jin. BUT I just read that according to the Tekken creators, that Jinpachi didn’t in fact have the DG within him, thus we are back to Kazuya being the progenitor of the DG…that is until Tekken 7 was announced. In this new upcoming Tekken 7 story, we are supposed to learn more about Kazuya’s mom, and it (I guess) has been hinted that the DG started with her as she was the first of the Mishimas to have the DG within her. So yeah, the whole bloodline of the DG is being turned upside down from games to game, as they are seemingly still trying to figure it out themselves.
Regardless, the DG is a power source of energy that allows one to augment their form to produce superhuman physical strength, as well as many other supernatural abilities. In the (now old) Tekken: Blood Vengence, it was shown that all the Mishimas (Hei, Kaz, and Jin) were all capable of great power, I guess just because they are Mishimas, as Heihachi was able to keep up with his son and grandson through just training and determination, but as Kazuya and Jin began to take it up a notch, they were clearly on a level beyond even Heihachi, blowing up buildings, flying around, as well as surviving blows that presumably would have killed them before their transformations.
All in all, I have to say that I don’t fully understand all the “Angel” and “Devil” stuff (even from way back in the day), and I honestly don’t think the Tekken creators have tried to explain it all, but it is clear that those who use these transformations gain a great power, but it does have a limit. That being said, there are various levels to the transformation process. In the Tekken: BV movie, Jin himself went through at least 2 transformations, the last of which trumping Kazuya’s full devil form, so it is wholly possible that there are more levels to this power yet to be shown.
In regards to comparing the DG to the SNH or Gargos’ power, the DG is WAY different, in that it doesn’t seem to be connected to any power source other than the host. The only other character officially to have been said to also use the DG was Azazel in T6, and still much of his story is shrouded in mystery. It was said that he is connected to / works for the Devil, and as such was the one who originally infected the Mishima family with the DG…most likely Kazuya’s mom, but yeah, we just don’t know yet.
Of the 3 power sources, the DG, SNH, GP…which one is more powerful? Well, as you can see the DG is really just as fleshed out as Gargos’ power, even though Tekken is now on its 7th game in the series. So I’d have to say that the SNH is more powerful, with the DG in 2nd, and the GP fighting for 3rd place.
Objectively, the DG has shown itself to be less powerful that the SNH as it has manifested in Akuma, but more powerful than the GP right now with the movie and all, but I think the potential for the GP to be more powerful is there. Not to mention, that I don’t find the DG, its current iteration anyway, to be that interesting overall. Transformations in themselves are cool and all, but I want to learn more about the origin of the actual power, which they haven’t tried to go into yet with any degree of detail.