Actual GDP: Just that
Forecast GDP: Actual + no further shocks
Natural GDP: Forecast + full utilisation (i.e. no current or residual shocks, either)
Potential GDP: Natural + fully flexible prices
Efficient GDP: Potential + no market power
That then gives three different versions of an output gap: Actual minus Natural, Potential or Efficient.
For some models, there is no difference between Natural GDP and Potential GDP. I don’t like those models.