Though this is a bit dissapointing, we do not fully know the truth. We do know for a fact that Guy's Father Duy was in the same situation as he was, but was weaker (the eternal genin). He learned the Eight gates from an unknown source. Duy claims it took him 20 years to master the eight gates. The only piece of knoledge we have on the subject is simply summarized to:
By undergoing intense training, one can learn how to open these gates allowing the user to surpass their own physical limits at the cost of extreme damage to their own body.
In the same paragraph on the wiki, the Gates
limit the overall flow of chakra within a person's body. The basis for the idea of the chakra gates comes from the body's limits on the functions within it. This makes the body much weaker, but it keeps the body from expiring too soon.
So we know that the Gates are simply, Literally, Chakra limiters, and Chakra flow increases strength, speed, and other physical traits. In the anime, it is shown that to open the gates, you need only Overload them, which forces them open. Therefor, it does make sense that one who spends years if not decades training intensely, like Duy or Guy, would eventually accidentally overload the first one. it is also possible to do so if they are in battle in a critical moment. Either way, Knowing there are gates (sharinan and byakugan can see chakra flow, so they can probably also see the gates) and learning they can be opened through force is probably all you need to open at least the first 7. Guy has been shown to open several at once, and for the first 7 only ever had to clench his muscles to do so. The first 7 can probably just be opened by sheer force which can be learned and mastered by training.
The problem then is simply the 8th gate, which is shown to require a puncture to the gate itself, which is located in the heart. The issue is simply how they could figure that out. As we know, there has only been one master of the gates at a time, so there are 2 likely senarios, either 1, they watch their master use it (Guy knew his dad was going to use the 8th gate against the 7 Ninja Swordsman) and witnessed it, realizing that was the key to opening the gate. The second idea is simply that they cant force it open, either through lack of training (to not risk accidentally activating it) or its just locked that hard, and the puncture works to make it easier to open. If you know about openin the first 7, it is completely understandable that you would figure out how to make it easier to open, though I doubt you want to puncture yourself every time, except when you do not expect to live long without doing so.
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.