Back in AD&D, gravity on the Astral Plane was subjective, meaning that "down" was whatever direction you currently believed it was.
Is this still the rule in D&D 5th edition sources?
The DMG, p. 47 states you move by will on the Astral Plane, but is mum about if there is any direction you would experience as down doing so.
A traveler in the Astral Plane can move by simply thinking about moving, but distance has little meaning. In combat, though, a creature's walking speed (in feet) is equal to 3 x its Intelligence score. The smarter a creature is, the easier it can control its movement by act of will.
However, according to the Astral Adventurers Guide, the Astral Sea is the part of the Astral Plane that is not a wildspace system ("The Astral Plane" p. 20). Wildspace systems are "where the Astral Plane overlaps with the Material Plane", so the Astral Sea is in a way the "pure" part of the Astral Plane. And it works like this:
The Astral Sea not only has gravity (see "Gravity Planes" above) but also breathable, comfortable air.
With Gravity Planes described thus:
[...] on the Astral Plane, gravity is an accommodating force, in that the direction of its effect seems to be "that which is most convenient." For an object the size of a planet or moon, gravity pulls everything toward the center of the body, meaning that creatures can stand upright anywhere on the surface, and dropped objects fall perpendicular to the surface they land on.
For smaller objects, such as spacecraft, gravity doesn't radiate from a point but rather from a plane that cuts horizontally through the object and extends out as far as its air envelope.
[...] A floating creature that enters the air envelope of a larger body is immediately affected by the larger body's gravity (such as that of a planet) or gravity plane (such as that of a spelljamming ship).
So only if you are nearby a larger object, you will be affected by that object's gravity, and that is the direction of down, because you will fall towards it. This however suggests that when you are outside of any object's air envelope, you are not affected by their gravity plane, and can define which direction is down by your own free decision: it does not matter, as there is no force working on you, you are moving by your will.