Please check ALL the conditions using Simbrief have been satisfied, Page 60 of the manual.
If you want to check what GSX is doing, just turn on logging, and activate a gate, there's a lot of diagnostic about SimBrief, telling all the possible reasons why your flight plan hasn't been used.
One thing that can be easily overlook is the airplane ICAO type, if you use the default profile for a 737-800 on Simbrief, the airplane code is B738, which is the official ICAO type for the airplane. However, PMDG has set the icao_model in the aircraft.cfg as "737-800", GSX obviously read the icao code from SimBrief *and* the icao code from the airplane, so it expects them to match, otherwise it would think the airplane it's not the correct type.
Unfortunately, the ICAO code in Simbrief airplane editor only accepts 4 characters, so I guess your best option would be editing the aircraft.cfg to read B738, or ask (gently) to PMDG if they would be willing to use the standard ICAO type in the icao_code field of the aircraft.cfg.