The technique is a result of Gohan's rage due to Cell's rampage, from both his Cell Juniors to his Perfect Form itself. It is used again in the movie Dragon Ball Z: Bojack Unbound, when Gohan faces Bojack in his full power state.
In the Budokai Tenkaichi series, Unforgivable allows Gohan to instantly regain all of his ki and puts him at MAX Power. Unforgivable uses three Blast Gauges. In Dragon Ball: Raging Blast, Unforgivable is a Super Attack that eliminates ki consumption from signature skills and increases attack power while draining ki when still in effect.