Gravity is a measure of how fast a falling character reaches their maximum falling speed. A character with high gravity does not necessarily have a fast falling speed; they simply reach their top falling speed faster. Gravity also affects how high a character is able to jump; two characters with the same initial jump velocity will not jump the same height if they have different gravity. Naturally, a character with higher gravity would jump lower than a character with lower gravity (assuming they have the same initial jump velocity). Gravity is measured in units/frame².
Gravity does not take a direct effect on vertical knockback in Super Smash Bros. and Super Smash Bros. Melee, although since a fighter's falling physics remain intact during hitstun, fighters with a combination of high falling speed and gravity will effectively resist vertical launch distance. From Super Smash Bros. Brawl onward, vertical knockback's velocity suffered by characters is now based on both their weight and their gravity (once they enter tumble); the formula to determine the gravity penalty is (g - 0.075) x 5 . This causes higher-gravity characters to be launched at a higher velocity, but horizontal launch speed is not directly affected. This reduces the natural effect of gravity on launch distance, keeping characters with low weight but high gravity (most notably Fox) from having disproportionally high vertical endurance, and in fact results in their endurance being slightly worse than a fighter with lower gravity, due to the formula overcompensating while not considering falling speed. Despite horizontal launch speed being unaffected, fighters with higher gravity also tend to have poorer horizontal endurance, due to the way the game decays launch speed. Fighters' total launch speed decays over time, with the horizontal and vertical components decaying such that they both reach 0 at the same time, and as such, when vertical launch speed is increased, horizontal launch speed decays more slowly. An example that shows the effect on gravity with knockback is with Zelda and Sheik in Melee and Brawl. In both games, Zelda and Sheik have the same weight and in Melee, Zelda and Sheik have the same horizontal endurance but Sheik has better vertical endurance due to her higher falling speed and gravity. In Brawl however, Zelda has better endurance both horizontally and vertically due to her lower gravity (although Sheik has slightly better vertical endurance with momentum canceling).
Gravity also affects how difficult the character is to combo - gravity does not take effect on knockback if a character is not put into tumble. By applying slightly more knockback to characters that more quickly fall into the next attack, it slightly normalizes the effect of setup and multi-hit moves on the cast. Particularly with non tumble knockback, characters with low gravity will be sent higher, while characters with high gravity will be sent lower. After getting hit by an attack, a character (on the ground) with low gravity may end up in the air after an attack, while a character with high gravity will land while they are in hitstun. The overall impact this has depends on the game. In Smash 64, Smash 4 and Ultimate, characters suffer a similar (or identical) amount of lag if they land during hitstun, with this affect usually altering possible followups, depending on where the opponent ends up after hitstun. A character with low gravity may only be vulnerable to certain upwards hitting attacks, while a character with high gravity could be vulnerable to numerous ground attacks. In Melee, the character will go into their normal landing animation if they land during hitstun, which can result in a character suffering from much less lag after an attack than they would have if they do not land. In Brawl, a character will go into their hard landing animation once hitstun ends, regardless of when they land during hitstun. This means that higher gravity characters will suffer from more lag from various attacks than characters with low gravity, putting them at a disadvantage.
A notable affect this difference in knockback has is that higher gravity characters are considerably more vulnerable to chain grabs in Brawl, and in Brawl and Smash 4, it makes moves with low knockback chain into themself and other moves longer on characters with higher gravity, especially when combined with Brawl removing DI against moves which do not put opponents into tumble (an example of this being Sheik's forward tilt, which can easily chain into itself multiple times).
In Melee and Brawl, gravity also affects how high a character will travel if they are aerial grab released. A character with high gravity will gain little height and will land quickly while a character with low gravity will gain a lot more height. Characters with very high gravity are at an advantage, especially in Brawl, as they will land before their grab release animation ends (although this puts them in a worse position if the grab release puts them offstage). Characters with very low gravity are also at an advantage as they will be released much higher into the air, making it harder for characters to chase them and followup in Brawl. Gravity no longer affects how high an opponent is air released as of Smash 4, however, gravity still determines when an air released opponent will land.
In Super Smash Bros. 4, a character's gravity can be altered by using the equipment bonus effects, Thistle Jump or Anchor Jump, with Thistle Jump decreasing it, while Anchor Jump increases it. However, both bonus effects also alter a character's falling speed.
In Super Smash Bros. Ultimate, while gravity still plays a factor on determining knockback, for launch angles between 70˚ and 110˚ characters now use a different gravity value that is linearly correlated to their weight, with lighter characters using a lower gravity value, and heavier characters using a higher one; though a few exceptions to this exist. As a result of this change, moves that deal vertical knockback are generally easier to survive compared to Brawl and Smash 4, thus making vertical survivability among the cast much more consistent than in both previous games. Additionally with non tumble knockback, this also makes it harder to combo characters who usually have high gravity, as they are sent much higher. Particularly, certain moves cannot chain into themselves nearly as many times against characters with high gravity. Conversely, Ultimate has the largest range of gravity stats in the entire series, including Melee, and as a consequence, the effect on horizontal launch distance is still significant. This can even lead to counter-intuitive results, such as Fox dying earlier horizontally from near the ledge than Jigglypuff, despite the former's weight being 9 points higher than the latter.
Super Smash Bros. gravity values
Compared to the later Smash games, Smash 64 has very low gravity values for all characters and as with most attributes, Smash 64 uses a different scale to measure gravity.
Super Smash Bros. Melee gravity values
Melee significantly increased the gravity for all characters and it is largely the main factor as to why Melee is a less floaty game than its predecessor. Melee also uses an alternate scale to measure gravity which would be reused in later games.
Super Smash Bros. Brawl gravity values
Much like with falling speeds, every character's gravity (aside from Sheik's) was decreased. Brawl also introduced the gravity penalty where gravity would be added into the knockback formula for vertical knockback causing characters with higher gravity to suffer from increased knockback once they enter tumble.
Super Smash Bros. 4 gravity values
Much like with falling speeds, gravity was increased across the board with most characters having higher gravity.
Super Smash Bros. Ultimate gravity values
Gravity values were once again generally increased across the board. Gravity values have also been notably changed for moves which send opponents at an angle between 70° and 110°; with heavier characters using a higher gravity value than lighter characters when being hit by moves with vertical knockback.