Say a wizard casts reverse gravity, if creatures with Flight are caught in the radius, are they affected by the pull?
Reverse Gravity states:
All creatures [...] that aren't somehow anchored to the ground in the area fall upward...
It says nothing about flying creatures being unaffected. Therefore they are affected.
To answer this question, in my opinion, you first need to establish the type of flight. Flight gained from "wings", flight gained through "magic", and finally, flight gained from "magic items"
Creatures with wings would be affected by Reverse Gravity, because they gain flight through pushing they wings against the pull of gravity. Reversing that pull would cause them to fall upwards in the same way as a creature without flight. The Dexterity Save would allow these creatures, rather than grab hold of something, to adjust their flight to the new gravitational pull. In other words, reverse themselves to fly upside-down.
However, creatures that use magic to fly, use forces to "defy" gravity, no matter if it is reversed or not. So they would be unaffected by Reverse Gravity.
And lastly, you have creatures that use magical items such as the "Broom of Flying", or a "Flying Carpet" to fly. The Reverse Gravity may not affect the item that is defying gravity, but it still affects the user of that item, because they essentially are now sitting on the wrong side of the item to be save from the gravitational pull. However, if a magic item were to give the user the ability to fly rather than fly upon, such as a "Potion of Flying", then they defy gravity rather than the item, and are thus unaffected by Reverse Gravity, just like the creatures who have their own means to defy gravity through magic.
This to me seems the most logical approach to ruling on the effects of Reverse Gravity on creatures with flight. However, in no means is this an official answer.