Eh... they flip in mid-air? If there's enough force induced on the gymnast's body to counter the pull of gravity (9.8 m/s^2) say, a trampoline on 8 muscular girls throwing her upward, or, she simply jumps up, then it will provide time for them to do flips in mid-air. As Connor pointed out, though, gravity will still pull them back to the ground.
Muscles. Gravity doesn't really make a difference- if you haven't noticed, they DO eventually hit the ground :P
they exercise a lot to keep themselves fit and their main reason for fitness is a routine diet and gyming
I have been doing wushu for about 2 years and it all depends on how your jumping