Hello,
I agree with you the process in not very easy/clear.
After several tests, the correct process is:
1. Import simbrief on OIS loadsheet page
2. Set ZFW
3. Set Fuel
4. Then you can select boarding on GSX.
Like this refuelling and boarding will be progressively and at the same time.
If you decide to start boarding first, ZFW and fuel cannot be set.
One more thing I recommend to use GSX directly from the toolbar in-sim and disable the GSX Remote control on the OIS.
