Hello Umberto,
I noticed something, GSX reads the data from Simbrief when the simulator is loaded. But if it is already running, i.e. GSX has already read in the data, but you then create a new flight plan via Simbrief, the data is no longer read in.
I assume that GSX does not monitor the Simbrief account and therefore does not recognize the new flight plan.
However, this means that neither the number of passengers nor the fuel are correct.
Is there a solution to this?