The following rules about gravity appear in D&D 5th edition:
- You take 1d6 damage per 10 feet fallen, to a maximum of 20d6 (Player's Handbook).
- If you're flying, and you stop, you fall (Player's Handbook).
- You fall 500 feet per turn (Xanathar's Guide to Everything , as per this question).
- The average man can make a long jump of 10 feet, or 3 feet up, with a run-up; a highly exceptional human can easily long jump 20 feet, or 8 feet up. (Player's Handbook). By comparison, the world record long jump is 29 feet, and high jump is 8 feet.
- The Ethereal Plane ignores gravity (Dungeon Master's Guide).
We can reasonably infer that gravity exists, and that its effect is approximate to that of our Earth, in that a five foot fall will not injure you but a ten or twenty foot drop will, and a 200 foot drop will almost certainly kill you; and the most exceptional human can jump eight feet straight up. Were gravity much higher or lower, that eight foot jump would be either impossible or so easy that everyone can do it, and people would either float safely down from rooves or crawl cautiously over deadly stairs.
However, we can also discern that D&D's rules are an approximation of gravity for fast gameplay. Realistically, terminal velocity for a human is 53m/s or 1,043 feet per 6-second round, and you take 12 seconds or two rounds to reach that velocity, at which point you have fallen 1,500 feet. Terminal velocity also varies by creature, so a cat has different terminal velocity than a human.
If you ignore that D&D rules are an approximation, the laws of physics don't behave as we understand them. The gods simply will all creatures to fall at the same speed, hitting terminal velocity of about 25m/s after 200 feet. In reality, it's an artefact of the rules being a simplification for gameplay purposes.
According to lore, the planets of Abeir-Toril (Forgotten Realms), Oerth (Greyhawk) and Eberron are all spherical, and approximately the same size as our planet. Where gravity is substantially higher or lower than our Earth, such as on another plane, the rules always describe it as such. We can assume that gravity works approximately the same way as our Earth, and that the rules serve adequately to adjudicate any gravity-based situation that occurs in normal play.