If I want to purely science this question *AHEM*
Reverse gravity would technically reverse the net gravitational force in that cylinder.
Because, as stated in the spell, "All creatures and objects that aren’t somehow anchored to the ground in the area FALL upward and reach the top of the area." Meaning that technically it follows the rules of falling under regular gravity.
In space (or I think the astral plane is the dnd equivalent?) though, the effects of gravitational force are seriously reduced. So, that net force, even when reversed, doesn't really amount to anything.
Which is how I would play it, because I interpret reverse gravity that way.
BUT
This could also be understood as like, "HHMMM, what I had a 'CREATE GRAVITY' spell, that I can use to technically 'reverse' gravity. Would be quite nice." In which case your thing might apply (and also whichever wizard made the spell like this is kinda dumb tbh).
Plus, taking (force?) damage due to a reverse gravity spell black hole sounds awesome.
AHEM
But's that's just my oPiNiOn.
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