I've noticed a flight starting on the PMDG 737 with 2 class pax, passenger count by GSX will only be the 1st class
To put it simpe, i've set up a flight plan in simbrief with the 737-800, and the total PAX for that flight is 160, so I set in the FMC PAYLOAD page, 8 pax in 1st class and the remmaing 152 in 2nd class... and when I requested boarding in GSX menu only a small bus with the 8 1st class passengers came up...
I don't know if it happens with the 700 and 600 variants... or maybe it really isn't a GSX issue...