Hi,
You're lucky that there are a number of nice gyms near the site of your meet. You should consider traffic patterns at the time you would need to drive to the gym—traffic in San Diego going out from the city in the afternnoon is limited to a few major arteries, and it can be quite slow. Accordingly, much depends on where you are staying. The hotels listed at the meet site are in Hotel Circle, just north of downtown San Diego. I’ve listed some anticipated travel times (courtesy of Google, but not in rush hour) from the meet site:
Victory, closest by a small margin, is a newish club with a small team in a nice facility. It’s about 6.4 miles (8 minutes) away from the met site. My daughter enjoyed her visits there.
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Mission Valley YMCA, which Bogwoppit recommended, is only 6.7 miles away (8 minutes) from the meet site. I thought it was nice facility when I visited there a couple of years ago, but my child hasn’t worked out there.
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San Diego All-Stars is also not far away, 13.4 miles away (about 17 minutes). My daughter has worked out there, too—and enjoyed it. This is home to a larger team.
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San Diego Gymnastics Academy is about 15 miles (16 minutes) away.
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If it was me, I’d consider the closest gyms and try to enjoy my visit and avoid traffic. (I'd probably go with Mission Valley or Victory, based on the proximity to the meet site and how easy it was to arrange a visit.) One or more of these clubs should welcome your daughter during her visit.