I LOVE IT!!!
Luffy looks like he came straight from those Japanese paintings that portray demons, gods and the like that always look angry.
Blowing up your muscles instead of your bones, clever. For a moment there I thought he was transforming his arm into a canon
It indeed looks a bit like Bellamy's DF, though I wouldn't consider it completely the same. A spring is something that easily gets compressed and then stretches again when released. Rubber however isn't a spring, it's just flexibel, so Luffy had to use an enormous strength to compress his limbs like that, but the strength released by it is far higher. In fact it has more in common with Kaku's Kilimanjaro tech than Bellamy's DF IMO.
And it seems my analysis of what was needed to achieve Gear 4th, was correct v
The latter is not necessarily true. Gear 2nd and Gear 3rd are indeed 'Gears' because they are supportive techs unique to a rubberman. However the fact is that Gear 4th has been introduced post-time skip, so if the same standard to qualify something as a gear is applied, he could have gotten Gear 4th pre-time skip. What defines the difference the most between pre- and post-time skip Luffy, is his skill in using Haki.
So combining those two together IMO Gear 4th would be something that only can be done by a rubberman, however the usage of Haki is mandatory as otherwise it couldn't be done even if you're a rubberman. As said before I personally would find it quite contradictory for Luffy's Gear 4th to be something he could have done pre-time skip.
I find it funny that someone actually suggested Nightmare Luffy, which Gear 4th quite resembles. I find it even more funnier how that it seemed some people already reached a certain consensus about how Luffy would become small (how that was supposed to lead to increased strength and speed, is a mystery to me), but in the end he grew larger xd
Gear4 owned as expected. Not particularly fan of the design(too much Nightmare Luffy like) but i like the Monkey-related moves. And Luffy stole another move from Sanji. At this point i wonder what Sanji can do that Luffy can't
Sanji's ability is what CP9 and many other trained marines can use, so if you consider that Luffy stole it, then likewise Sanji stole it. Also I'm not sure if it's the exact same principle. Geppo works by kicking the air repeatedly at extreme high-speeds, while with Luffy I think it works by kicking the air only once each time, but simply with such force it blasts him away. So one is based on speed, the other on power.