When GSX is used with an airplane that has been flagged as not using a default system ( "use cargo/fuel page" check DISABLED in the airplane configuration page ), GSX will regularly check the total fuel quantity and, as soon it detects the fuel increase stopped, it assume the airplane has finished refueling.
So, it seem the airplane must have paused its own refueling for enough time so that GSX considered it done.
I'll ask Aerosoft about this, maybe they are simulating something that caused this pause.