The takeoff roll begins when the airplane is lined up on the active runway, is cleared for takeoff, and power is applied. Has nothing to do with the gate.
Wake turbulance from a 747 can be very violent. Essentially a level 5 tornado is generated by each wingtip. The winds were not too strong that day, so the vorticies would have persisted.
Wake turbulence is very strong, but its effect on an A300 shouldn't have had this impact. A G4 or Citation sure, but an A300?