Boarding passengers while refueling is not so common, it usually requires calling the airport Fire Brigade be prepared to do an emergency evacuation, so we decided to stick to the standard, and not allowing it.
Also, since refueling from the left would likely cause a conflict between the passenger stairs, and refueling from right will cause a conflict with the baggage loaders, it's best not do both at the same time, the GSX fuel truck will wait for any loaders to complete.
boarding passenger while loading baggage/cargo is how it is now, since for GSX they are part of the same operation (Boarding)