The Gainer Flash is a backflip of one leg that is swung into. You don't need to know how to do a backflip first, as you can slowly progress into this trick, going from vertical to horizontal and then finally to performing it fully inverted.
The easiest way to progress into this is to think about it as an Aerial that is swung into.
You can also learn the 'cheat gainer' first, which is a Gainer Flash that is kicked over the side, basically a Swing Butterfly Kick.
The term 'gainer': In Freerunning and many other sports, the term 'gainer' is used to describe a backflip that is traveling forwards. A backflip that is 'gaining' distance. In tricking however it is used to describe any kind of backflip off one foot. This leads to a lot of confusion between trickers and other athletes - for instance when Vellu landed Trickings first Double Gainer and many didn't want to acknowledge it as a real gainer.