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Since Reverse Gravity is a concentration spell, what would happen if someone casts it, and then an enemy casts meteor swarm on the area in which reverse gravity is in place?

Now, meteor swarm is described as instantaneous, but still specifies it's blazing orbs of fire plummeting to the ground, as meteors do.
So, would the reverse gravity spell prevent the meteors from working, decreasing how they work, or have no effect?

I'd like both an answer that makes the most sense with the rules, as well as an answer that would make the most sense in general for a DM to implement. Personally, I was thinking that the fairest way of doing it would be to half the damage from them.

(This could also be asked for other spells that involve hurling objects, it's just meteor swarm, in particular, I was wondering about)

Simple answer: No

Reverse Gravity affects creatures and objects, and a spell is neither (though a spell may conjure or create a creature or object).

And, as you say, any spell with a duration of instantaneous effectively happens without any chance for something to affect it unless specifically noted.

A more 'physical' argument against Reverse Gravity affecting Meteor Swarm or similar spells is that the spell creates the blazing orbs of fire with enough force and momentum for them to reach their targets before gravity can have much of an effect.

After all, even without Reverse Gravity, a spell that launches something at a target doesn't have to worry about gravity pulling it to the ground before it reaches the target!

The blobs of fire created by Meteor Swarm would be affected by Reverse Gravity

As mentioned Meteor Swarm has a duration of instantaneous. For the purposes of the rules instantaneous has a specific meaning in relation to the spell duration:

Instantaneous

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.

The instantaneous duration refers to the creation of the objects in the spell, the "effect" of the spell. Importantly it does not refer to the resolution of those effects (and in particular the damage caused by the effect).

In the case of Meteor Swarm the effect it creates is

Blazing orbs of fire plummet to the ground at four different points you can see within range.

The key word here is "plummet". This is not a defined game term, thus we must check what it's natural meaning is in English.

Plummet means

to fall very quickly and suddenly:

House prices have plummeted in recent months.

Several large rocks were sent plummeting down the mountain.

She plummeted to the ground.

Falling is something that is explicitly affected by Reverse Gravity

All creatures and objects that aren't somehow anchored to the ground in the area fall upward and reach the top of the area when you cast this spell.

The orbs of fire are pretty clearly objects and thus would be affected by Reverse Gravity when they come into the area of effect of Reverse Gravity.

If you're following the rules to the letter, as above has said, "No"

If you're having fun then I'd say "Yes". Meteors don't propel themselves generally, their speed is all due to gravity. If we're talking flavour and have a flexible GM, I'd say the spell would cause the meteors to fly upwards until the spell ends, once it does they'll come crashing back down, effectively causing a time delayed rain of destruction on the initially targeted area. This could be used for some exciting setups where you combo the two spells before baiting enemies into an area for a nice surprise barrage.