Jack, 14, won Tuesday night's public vote to make it through the semi-finals.
"It's just ridiculous - all those comments were better than I could ever have dreamed of when I went on stage," he later told Britain's Got More Talent's Stephen Mulhern.
"I went on stage nervous, shaking with the microphone, feet stomping for no reason - like I was about to do tap dancing ... which I'm not very good at by the way".
Gabz, also 14, was in the top three when it came to the public vote and was chosen for the final over little Irish lads Jack and Cormac.
She's already won by getting this far.
Noel Gardiner, Gabz's dad
"I'm just feeling really, really happy - I can't wait for the final, I'm just so excited," the singer/songwriter declared, adding that getting in the charts would "just be the best thing ever".
On This Morning, co-presenter Eamonn Holmes marveled over the wealth of talent among such young contestants.
"Are you aware that you guys are different and that this is an amazing gift you've got?" he asked.
"What you don't know is we're part of an MI6 genetic engineering thing ..." came Jack's deadpan response, reducing Eamonn, Ruth and Stephen to laughter.
In his semi-final act, Jack made the most of the opportunity to poke fun at head judge Simon Cowell and it seems another judge may be in the firing line in the final.
"Who are you going to pick on out of the judges in the final?" Stephen asked. "Come on!"
"I can't say that!" Jack protested.
I was away at the time (when Jack told him about entering BGT) - that sounds bad. I was only away skiing, I wasn't in prison or anything ...
Matt Carroll, Jack's dad
New material from Gabz also looks likely, as she's keen to further prove her song-writing credentials.
"I think I'm going to do a new one because I have written more than one song and I want people to know that," the teenager said.
Jack and Gabz will join brothers Richard and Adam Johnson and 11-year-old singer Arisxandra Libantino in the Britain's Got Talent final.
The third semi-final will be aired on Thursday at 7.30pm on UTV.