With the Gravity effect in play, Flying types are vulnerable to ground type attacks, and will be neutrally effective when hit by a ground-type attack.
For example, if Talonflame gets caught in Gravity, a ground-type attack will be super-effective against it (neutral to flying-typing, but 2x as effective against its fire-typing). Vice verse, if a Pokemon like Jumpluff gets caught in Gravity, a ground-type attack will be not very effective against it (neutral to flying-typing, but 0.5x as effective against its grass-typing).
So, flying types who double with a Fire, Electric, Poison, Rock, or Steel typing will have ground super-effective against them. Flying types who double with Grass or Bug typing will have ground not very effective against them. All other flying-typing will receive neutral damage.
Source + Experience
Hope I helped! :)