The Gomu Gomu no Spear is another of the many trademark Monkey D. Luffy techniques from the series One Piece. This attack is one of the staple moves Luffy uses to show off his versatility with the powers of the Devil Fruit in his rubber arsenal. Literally, it is written as "Gomu Gomu no Yari" in romanized characters from the original Japanese version, meaning "Spear" in English. The attack has been romanized as "Gum-Gum Spear" in the VIZ manga and both the 4Kids and Funimation dubs, keeping titles for Luffy's other rubber-based attacks similar.
In performing the Gomu Gomu no Spear, Luffy will keep both feet together and kick out simultaneously with his toes. This very unique method of attack granted the extension of Luffy's legs to that of a spear, hence the name of the technique. The move, however, first found great prominence during Luffy's fight against Kuro, where he applied it effectively after dodging one of Kuro's striking attacks. This early application of the technique showed its utility both as a weapon and a defense technique.
As Luffy's capabilities grew further in the series, so did his techniques. At any rate, a couple of instances of his same powered-up attacks—though maybe they were not directly mentioned—were shown for post-time skip Luffy, who had developed improved versions for his Gear Second state. Of course, this applied conspicuously to the Spear among many others, which—clearly closed—left open the possibility of a "Gomu Gomu no Jet Spear". In the anime, there was a similar attack called "Gomu Gomu no Jet Lance" against Blueno, which provides clear evidence that Luffy has truly adapted this technique for his power-up states.
The trend for naming Luffy's attacks is a convention that usually goes "Gomu Gomu no" followed by a descriptive term that mostly links back to weapons or certain close devices that his rubber body imitates the function of. This would suit the Spear technique, considering the piercing and extending characteristics of the attack.
Which is where the real debate about the Gomu Gomu no Mi, the Devil Fruit which gives Luffy the rubber, comes in. What was once considered a standard Paramecium fruit had its nature and history speculated upon greatly after several series' revelations. However, whether it is classified as such, tactics like the Gomu Gomu no Spear convey Luffy's ingenuity as far as how weaponizing his elastic nature goes into combat.
But that is not all, for again, just like most of Luffy's techniques, the real skill of the Gomu Gomu no Spear is not really in its power but in its unpredictability. The price of this uncontrollable add-on to the range of tools in his fighting style leaves opponents off guard who are not too familiar with Luffy when his limbs suddenly stretch out.
Since the series never seemed to have an end, Luffy continued to innovate and combine his techniques. The basic Gomu Gomu no Spear would remain a part of his moveset but had likely been superseded by more advanced techniques in high-level battles. Nevertheless, it stands as testimony to Luffy's earlier creativity in adapting his rubber body to combat.