The only time that I ever was able to see practices for the gala at US Nationals or Skate America was when I was credentialed as a USFSA official or as a chaperone to a skater participating in it. I would be shocked if admission policies have loosened since then.
Honestly, you're not missing anything. Because there is no public admission and these aren't programs that will be judged, it tended to be very informal with skaters often participating in sweats as opposed to costumes and hair/makeup like an official practice. There wasn't any formal skating order or special time for run-throughs by discipline, skaters tended to show up and leave on their own schedules.
Honestly, as a skater myself, I’d enjoy watching a “normal” practice session where everyone is more relaxed. But it makes sense it’s not open to the public. It kinda ruins the “surprise” of the gala programs if you’ve already seen the practice run-thru’s of said programs.