GB got 4L fluid.

when they change, they usually just change the bottom part which contains 2 L. ( it's actualyl partialyl change, still got the old fluid inside)

Full flush GB oil change would take longer cause more troublesome

Not quite, that is their standard procedure when they say clean ATF change, the bottle u bought, is also most likely 2L only too. Which they just replace the bottom part of the tray.

U can try to ask them to do a full flush change, need 4L bottle. Of ATF fluid.