Hi,
I have the exact same issue.
It seems that what is entered by GSX into the FMC is incomplete (for instance, if Simbrief says 5600kg of fuel, GSX will just type in something like 531 or anything with only 3 figures), and if by doing so (if you enter something <999kg) it will automatically multiply this by 100 and so it will saturate to the max fuel the 737 can have (so 100% of fuel load).
But still, there is a bug somewhere, that coincides with last update of GSX pro so... Hope that this will be sorted out soon.
Thanks