Yes... your GTX has a 15 gal tank.
As far as "How much is each bar"... there is no way to say.  Unlike a car... it can be sitting nose high, or low... and there isn't any baffles in that tank... so as the fuel sloshes back and forth... the level will change.
Think of the gauge as an intelligent idiot light.  When it gets close to the bottom... it's time to put fuel in. (or switch to reserve)