You can get get while they are filling up the gasoline tanks however it's not recommended and you really should not fill your gas tank up or get gasoline for your vehicle when the gasoline tanker truck is filling the gasoline tanks.
When the gasoline tanker truck is dropping gasoline into the gas stations gasoline tanks sludge, possible dirt and other debris can get stirred up in the tank which then gets into the gas pump when you pump the gasoline into your vehicle.
Then that debris can get into your vehicles gasoline tank and then clog your fuel filter and fuel injectors as well as possibly damage your fuel pump.
If the gasoline station is having their gasoline tanks filled you should wait until they stop filling the tanks and then get gasoline or go to a different gasoline station to get gasoline for your vehicle.