Im 29 had a venus bypass crossover graft 2 weeks ago, won't know if it's worked till i go bl but i need to know if i would be ok going back to the gym?

Not yet...: I assume you mean they used veins to bypass coronary arteries, if so you need to wait six weeks and then have a stress test. This is followed by cardiac rehabilitation, which i would strongly recommend, especially at 29 Y.O. If you mean they bypassed a peripheral artery or vein, you should wait 6 weeks until the graft is healed before exerting the kind of pressures heavy exercise will bring on.