Okay I wonder if im getting this right.

A Glabrezu can as part of their multi-attack make two pincer attacks as well as cast a spell.
Could he then possibly make two pincer attacks, grappling two people, cast fly, fly up 60 feet and then drop these players for 6d6 falling damage. Or even in their next turn fly up another 60 -120 feet and drop them for a possible 18D6 falling damage?

This seems a little overpowered, ofcouse their concentration could be broken but that would still cause the players to fall down as well.

A glabrezu is only Large, so while it drags medium creatures its speed is halved. Additionally, the party should be at least 5th level by the time they fight one of these, so there should be a spellcaster able to counterspell the glabrezu’s 1 a day fly.

It’s powerful if the party let it do its thing, but if they take some precautions it shouldn’t be too bad. (Precautions include not being dwarves)

If it multi attacks, that’s it’s action. Casting fly, even innately is also an action. It couldn’t cast fly until it’s next turn. It still could try to pull it off, but that would at least give the players a chance to escape the grapple. 
Now, if it had already cast fly before the fight, it could have a chance to do that, but fly is a concentration spell, so that would be a risk. 

Part of a Glabrezu's multiattack is using two pincer attacks which grapple the target if it hits as well as cast a spell. You could thus make two pincer attacks, then cast fly as its multiattack and then use your movement to fly up to 60 feet. Possibly at half speed because of having to drag someone along.

So the glabrezu would be up at least 30 feet before the person would get their first chance to escape from the grapple.

You have to bear in mind that a creature can only carry another creature within the bounds of its strength. It has a strength score of 20.

Using the normal basic rules this means it can carry 300 lbs and no more. If the combined weight of both pincered creatures is equal or less than 300 lbs, the Glabrezu can carry them while flying but its speed is halved. If the weight is greater than 300 ft the speed drops to 5 ft and would then be halved for grappling to 2.5 feet, which is not enought to cause falling damage.

Using the variant rules the Glabrezu will be heavily encumbered if carrying 200 lbs or more, which would be likely if carrying two creatures, which means it loses 20 ft of movement which brings it down to 40 ft fly speed. The movement is then still halved to 20 ft. If the weight is greater than 300 lbs then the Glabrezu cannot move with them.

The halving from grapple still stacks with speed reductions because it represents trying to move an unwilling target. If a creature is willing you don't halve the speed.

With the above in mind, yes it is valid for the Glabrezu to pincer-grapple, fly up and drop. Likewise, your character with flying and extra attack would be able to do the same. So, giving the fighter or barbarian a fly speed is very useful...

Ahhh, my mistake. That's what I get for not reading all the  way to the end.

 It is a large creature, so it's carry weight is 600.