The Fuel integration will be completed after the release of the big Fuel Update, we have been working on for a some time, to support 737 and lower wing airplane with a complete fuel crew.
So, right now, nothing really changed compared to before: it will still your responsibility to ensure the fuel quantity is lower than the required one when you call the GSX fuel service, the only difference is that now, instead of starting with a random quantity, you are starting with the correct one (that is assuming you pressed "Reload Simbrief" before calling the GSX Truck), so it might feel a bit redundant.
In the final version, GSX will automatically lower the quantity before the GSX trucks decides if you need fuel or not, and will reset it again to the planned value to trigger the GSX fuel animation.