Luffy's Weaknesses have become his Strengths - His Gears were knowingly or unknowingly used to train his body
When he ran into CP9, he realized he needed to be faster. After his first skirmish, he figured out a way to bypass the years of training it took CP9 to accomplish their speed. But, the doping shortcut he came up with came with the high price of potentially shortening his life... And, he could not use it extended periods of time. Gear 3rd, is an ability that allows Luffy to turn Air into Mass to allow Luffy to deliver a more powerful blow. While not taxing his body to the point of shortening his life, it still came with a very significant drawback. It shrinks his body after use and the lag time before returning to normal was too long and could cause him to lose his life against an opportunistic foe.
Pro - Amazing speed, increased power via (force = mass * Acceleration) emphasis on the acceleration
Con - Shortening life, extreme strain, and no stamina
Training - By consistently using this method, Luffy is becoming more and more used to operating at this speed. He stated that he did not know that this way to move was possible, but it was not something he could achieve without the doping. For now.
Pro - Massive increase in power and damage Same equation but with emphasis on the Mass.
Con - Slow, short duration, and shrinking
Training - By consistently using this gear, Luffy was getting acclimated to his devil fruit. Removing two of the weaknesses of duration and shrinking.
When fighting Doffy, Doffy pointed out two of his gear's remaining weaknesses. 2nd makes him fast, but is too light. 3rd packs power, but because of how slow of an attack it is, it leaves him wide open for attacks. Incomes Gear 4. Now, if we follow the pattern, Luffy isn't strong or fast enough to beat his current opponent. While he did take 2 years to train, what he came up with first was a shortcut to gaining explosive power and speed in an instant. We all assumed that this was the next level however, it was only the next level of training. As pointed out by Rayleigh, Doffy, and Law, Gear 4 overused haki...
Pro - Exponential increase in Speed, Power, and Elasticity, Defense (armored and rubbery) whereas Gear 3rd left him open.
Con - Duration and Haki Drain and completely exhausted for 10 minutes. Similar to Gears 2nd and 3rd, plus and added one with Haki Drain.
Training - By consistently using this form, we see that 1 Luffy is able to use it for longer periods of time which means it must have been increasing his Haki Stamina! But, tons of haki still can be depleted when used this way.
So, through the use of gears, Luffy has increased his speed, power, and level of damage. However, at the highest gear, the duration and haki expenditure are still weaknesses.
Fight with Katakuri.
During this fight, it is apparent that Luffy was aware of his weaknesses so he attempted to fight Katakuri until he could expose a weakness. Once that "weakness" was exposed, he planned to go to his "ultimate" shortcut form to deliver a finishing blow. But, this didn't work against a guy that could see the future. The best part about this future sight is that it requires you to remain calm, allows you to be more efficient, and can be used to increase your offense and defense!
Pro - Ability to see the future is the ultimate defensive technique. Also, helps tremendously with offensive counters.
Con - not really... But, you need to be calm in order to use. As seen the first time Luffy fought Kaido, he was infuriated and unable to think clearly, see the future, and avoid the blow.
Training - Some via Rayleigh, but fully unlocked Future Sight during the battle with Katakuri. So long as Luffy remains calm, this defensive should be permanent.
Alright. How does Luffy stack up now? He has a ton of Haki now but doesn't know how to utilize it efficiently, He should be getting faster in his base form from training in gear 2nd. Still not strong enough to defeat stronger opponents as shown by Big Mom nerfing his Leo Bazooka and Kaido nerfing his King Kong Gun...
Enter Udon and Hyogoro
In Udon, Luffy undergoes extreme weight training. Not only does he have to haul enormous boulders, he races with them and carries more than one at a time all while being weakened with sea prism. This increase in strength will be utilized in two ways. Then, Hygoro trains Luffy in a more efficient and more potent use of haki. We have seen that Luffy has increased his haki stores, but Hygoro mentions that Luffy does not need to be using all the haki in his body at the same time. He is taught how to train himself to allow his haki to flow from parts of his body he doesn't need it and to concentrate it all in one point. The point of attack.
The best part about the strength training and the haki training is that they are being done in base form! Now, how does this strength and haki control complete the Luffy we have grown to know and love?
Strength: Now that Luffy is faster, he still needs strength. Gear 2nd was still slow because he was not able to move the massive arm as fast as he could his regular pistol or red hawk sized arm. Getting stronger in Udon has allowed him to be able to now remove the 3rd and final weakness of gear 3rd while allowing him to keep the increase in power and damage dealt.
Ryou: With tons of haki being gained from increasing his Gear 4th endurance, being able to control it more efficiently and being able to direct it to the point of attack makes his base form and Gear 3rd form punches pack even more of a punch. He is not wasting haki and is being more efficient with it, so it'll last longer. This rids him of the endurance issue as wells the 10min haki drain.
By using the gears for trading, Learning Future Sight, and learning Ryou Luffy has gained more strength, power, defense, and lets say equal speed in his base form that he had in his previous ultimate form! Where previous ultimate attack, Gear 4th King Kong Gun, was nerfed by Kaido, a base gear 3rd, with future sight for defense, added strength for speed of moving the massive hand, and finely tuned haki for an explosive impact, has surpassed the the former attack in power while covering up all weaknesses!
Could it be that instead of a Gear 5th, we have gotten a Gear Zero instead?