The last thing to think about is the Moves he uses. Well like almost every move besides Rasen Shuriken were invented off screen. By who and what we do not know, But what is clear is that Guy can not use Ninjutsu, as well as Lee. Every Jutsu they used was some kind of Taijutsu, usually a simple punch or kick, sometimes enhanced with the power of the Gates.
Morning Peacock was simply punches so fast that the air friction alone set them ablaze.
Daytime Tiger merely used the strength to compress a massive amount of air into a very small point, then release it in a massive shockwave.
Evening Elephant was to create air cannons with your punches
Night Guy Was nothing more than a Chakra enhanced Super kick.
In the end, all these moves are Simple punches or kicks, which could be performed at lower or any level, with or without gates, but with reduced effectiveness. Like most forbidden Jutsu, They were also likely written down in scrolls, recorded as useful/effective moves, but with high risks attached. It only takes one time to test a move, and one survivor to record it. We also know Might Duy had reached all 8 gates, so he could also have had time to invent some of these moves. Beyond that, We are just looking at fairly simple moves, for a guy that is undeniably a Genius Taijutsu specialist, and only 2 of them are doable in the 8th gate, both of which are just a punch and Kick, simple moves in principle.