If you didn't know, Flying Press is the only dual typed move in Pokemon being a Fighting/Flying type. Dhelmise/Trevenant/Gourgeist is a Ghost Grass type thus making them immune to Fighting, but weak to flying. What will happen if you use it on them?
It doesn't work
Also if you click in the flying press move on this website they'l show the own chart of flying press. So you can see how he works with another types too.