How does Gravity work?

Question

I heard that if Gravity is used, all flying-types, levitating Pokemon, and all using Fly, bounce, etc. are brought down, and certain moves aren't allowed for 3 turns.
Now this is a scenario between a Foretress and an Aerodactyl.
Turn 1
Aerodactyl used Fly!
Fortetress used (a random move)!
Turn 2
Foretress used Gravity!
(This is where Fly is supposed to hit, but what happens here?)

Comments

So they have the same speed? Since one turn a-dactyl goes first, second turn Foretress goes first. Just making sure in case I an test it

Answer 1

Fly fails, and the semi-invulnerable state ends.
source
(note that I didn't use iron defense, but I don't think it makes a difference)