The second one.
I think its possible you don't know how counter spelling works. Counter spelling only occurs as the spell is being cast, by another caster who has readied an action to counter spell, correctly identifies the spell being cast, and casts an appropriate counter spell (like a second Reverse Gravity, because a spell can always counter itself). If all that happens, the spell doesn't take effect in the first place.
If Reverse Gravity is cast by two different creatures in overlapping areas, does the second casting restore normal gravity (countering the spell) or does it simply last for the longer of the two durations? Reverse Gravity doesn't list a counter spell.
Since the spell doesn't actually reverse gravity (you can't fall to the moon for example) and it uses the wording that it causes things to "fall upward" I'm assuming you can't counter it.
It all depends on what the spells mean by Gravity. Does it mean the Gravity of the Plane itself or does it mean the current Gravity on the area affected by the spell?
Basically DM's territory
Casting the spell reverse gravity in two overlapping areas only overlaps the spell effects. Both are implemented as if the other didn't exist.
The overlapping areas would use the stronger effect. (aka overlaps, does not stack).
