It’s hard to say when the Global 6000 might be added. But for now, you could always create your own custom airframe using the My Fleet section.
I recommend choosing a similar aircraft type to start (for example, maybe the GLF4), and then updating the name, weights, max passengers, fuel capacities, etc. to match the Global 6000 specs. Finally, you can increase or decrease the fuel burn as needed using the “Fuel Factor” option.
This part can take a bit of trial and error, but initially, I recommend you leave the fuel factor at zero (“P00”), do a flight, check whether you burned more or less than what SimBrief predicted, and then set the Fuel Factor accordingly to correct it for future flights (if your airframe burned 10% more than what SimBrief predicted, then you would set the fuel factor to “P10”, or “Plus 10%”).
You should be able to get relatively accurate results using this method I think. Let me know if you have any other questions!