It’s possible that the “gnashing of teeth” (Mt. 8:12 NASB) might refer to regret, bitterness, or anger with God, rather than “gnashing of teeth” in some sort of physical pain. One reason to believe this is Acts 7:54. In this section, the Pharisees were “resisting the Holy Spirit” (Acts 7:51 NASB). And, they were “gnashing their teeth” (Acts 7:54 NASB) at Stephen’s preaching of the gospel. Perhaps, the gospel writers were using the same imagery to describe the regret, bitterness, and anger of those in hell. In this picture, they are torturing Stephen, and they are “gnashing their teeth” at him, rather than “gnashing their teeth” at being tortured themselves.