Gass was performing alongside JB in Sydney, Australia over the weekend -- and when Jack asked him what his birthday wish was onstage ... KG said, "Don’t miss Trump next time" -- a clear reference to Thomas Matthew Crooks firing a shot at DT over the weekend.

Sources with direct knowledge tell TMZ ... Jack wasn't aware Kyle would say this, and that he doesn't condone violence.

You can hear the crowd mostly laughing and cheering in support of his 64th bday wish ... but some people do seem to express shock as well, with one guy audibly saying "woah."

It wasn't long before Kyle's comment reached a different crowd online -- and they made it clear they didn't share his POV ... in fact, plenty of people are downright pissed about this.

As you can imagine ... tons of folks are condemning this -- saying it's a green light for further violence, and we've learned Jack himself feels the same way.

Safe to say ... he doesn't share the same POV as Kyle -- although it's unclear if they talked about the remark after the fact. And remember ... Jack's a Biden supporter -- he recently went and got onstage at a pro-Biden event ... and made it clear he'd be voting for him.

Trump’s gotten a wave of support from other celebs after the assassination attempt. From Elon Musk to Amber Rose to Larsa Pippen, they’re all showing they’re team MAGA more than ever ... but clearly, some stars are still very anti-MAGA despite what went down.