I don't think they were particularly efficient or common. The overwhelming majority of the less-than-eleven-million who died likely did so of the usual causes of death for prisoners in other labor camps in the 20th century and before - starvation, cold, and exhaustion.

None of this is intended to discount the evils of mass incarceration of innocents or their murders, by whatever method. Doing that to one human being is morally intolerable. Seven million or eleven million makes little difference - they're already on the wrong side of the only line that matters.