I played for the GT band once upon a time. This is not a defense to CGC but high chance he did not know what the band was up to during halftime. It's not like band and football team is communicating during the week. The band puts in their practice for the marching show during the week and when there are special days like 9/11 tribute, they practice a different show. The most contact the band ever had with the football team was when we went for practice one day and the coach was on the field and we all said "Hi, Bill Lewis!" and he waved back from a distance. I'm sure CGC didn't know what the band was up to.
Also, during the 20 min of halftime, each side is given 10 min to do their shows. I was not at the game (nor do they broadcast halftime shows) so not sure how the setup was. Sometimes, the opposing side has no band so the show finishes early and that's when you usually see the football team back out on the field, practicing and such. If you push close to the time limit, you'll see the band do their show and then run off the field instead of marching off. Maybe GT team is used to having a few min before 2nd half to stretch and do some FG practice? I'm just saying.
Edit: sorry, each side is given less than 10 min per side but don't remember exactly how long.