Yes, it's written in the manual. IIRC the departure airport and the aircraft type need to be the same between the game and simbrief (like "KJFK" and "A20N"), also the departure time in simbrief must be AFTER the time you start your flight (e.g. departure time 10:00 UTC, you have to load the flight with a time of day before that, like 09:45 or 09:30).
If it was for me GSX could just do away with those restrictions and just load the pax number from the newest simbrief flight, whatever it is.