No, g-fuel is completely based around the caffeine that's used to make it.
Your reactions are how fast your brain sends signals to other parts of your body and it's almost completely based upon your genetics. Taking Gfuel will not make you good at gaming, in-fact it probably wont even make you any better. Things like what weapon you use, your internet speed and how much sleep you have are far more important on deciding if you'll actually win a gunfight or not.
As I said in the beginning, Gfuel is basically just based around caffeine which will make you feel more energized and awake, however after those effects wear off then you'll feel more tired and exhausted than you died before taking the Gfuel.
If you're taking it to get better at Xbox games then it will honestly do nothing, the Xbox controller is designed in a way that means it's almost impossible to build muscle memory and the skill cap is generally quite low. That's because Xbox games don't have competitive ranks like CSGO or LoL and they don't want their younger fan base to be stomped on by better players. I've been playing COD since I was about ten and I'm eighteen now and I'm not one of the best players in the world, even though I've been playing it for eight years. I bet if I had of put those eight years into something like archery or League of Legends then things would have been very different.