Mico. we all have had that issue. but there is a fix. it's working now. I did 4 flights (2xPMDG738/773) with full services and it was perfect.
The only difference I've noticed is that with the 737, GSX doesn't overwrite my payload in the FMC setting, but with the 777, my entries are automatically overwritten by GSX as soon as I start boarding. For example, I entered a payload of 5,000 kg into the FMC via a simbrief, and during boarding, GSX adds another 5,000 kg to that, so that I would have 10k. Thats why I set the 777 airplane to 'Empty'
best regards and merry xmas!