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 jump force 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 jump force).
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. This also makes characters with higher gravity considerably more vulnerable to chain grabs in Brawl, and in Brawl and Smash 4, it makes moves with low knockback chain into them-self and other moves more reliably on characters with higher gravity 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 will end (although this puts them in a bad position if they end up off stage). 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).
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, characters now use a different gravity value that is linearly correlated to their weight, with floatier 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 SSB4, thus making vertical survivability among the cast much more consistent than in both previous games.
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 (except for 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.