Gravity is a measure of how fast a falling character reaches their maximum falling speed. A character with high gravity may not be a fast faller, but reach their top falling speed faster. However, its a factor in determining a character's accelerated falling speed due in part to its combination with a character's regular falling speed.
The vertical knockback suffered by characters is based on both their weight and gravity, with higher-gravity characters taking slightly more knockback; horizontal knockback is unaffected. However, the affect on gravity is minor.
Gravity also affects how high a character is able to jump; two characters with the same jump force will not gain the same height due to the different gravity each character has. Gravity is hardly noticeable in Super Smash Flash due to the its physics, while it is very noticeable in Super Smash Flash 2 due to physics being closer to how it is in the main Super Smash Bros. games. Gravity makes it easy to meteor smash any character.
The above was most noticeable for Tails in demo v0.9a, whose gravity was pulling him down while he tried to recover. However, he was not susceptible to combos and chain grabs that were notable for characters who had above average accelerated falling speed.
List of characters gravity values
|Zero Suit Samus||1.4|
Mr. Game & Watch