Jack says he was blindsided by Kyle Gass' remark on the heels of the shooting. KG made the comment at their show in Sydney over the weekend, as he celebrated his birthday onstage, saying he hoped another would-be assassin not miss the target next time. As you can imagine, the remark caught a ton of backlash.

Black says he would never condone violence, hate speech or encourage political violence of any sort.

Gass' comment -- in which he said, "Don't miss Trump next time" -- understandably triggered a firestorm of criticism, and it seems Jack had no choice but to put the tour on ice.

KG just issued an apology ... "The line I improvised onstage Sunday night in Sydney was highly inappropriate, dangerous, and a terrible mistake. I don't condone violence of any kind, in any form, against anyone."

He went on ... "What happened was a tragedy, and I'm incredibly sorry for my severe lack of judgement. I profoundly apologize to those I've let down and truly regret any pain I've caused."