They aren't defying gravity. They are simply stunts & tricks.
Skateboarding does not affect gravity gravity affects skateboarding. Many defying gravity move is just a trick earn from hard practice.
The plural of defy is defies. As in "this defies the laws of gravity".
Hovercraft - they defy gravity.
It doesn't, nothing can defy gravity.
Skateboarders do not defy gravity. They use typically an ollie to do aerial tricks. An ollie is a move in which the skateboarder "pops" the tail (to push the board in the air) and slides their front foot forward (to balance the board). This is probably the most useful trick in skateboarding. From an ollie you can grind or grab or flick the board.
No
The chemical that is called defying gravity.
Ultimately, no.
They cannot.
No they don't.
adhesion.
The water seemed to defy gravity.
No, they don't.