My initial reaction is there's a price for everything. In gymnastics, the price is the family's sacrifices, the "taxi mom's" time, the $$$$, and one last thing called "renewal." I don't know your child's age, level, or sense of purpose, so all I can suggest is that you get something in return that's worth the "price." Each person has a point where their "returns" for the week seem to fade due to stress, fatigue, and frustration, so is the added "price" going to create a "net gain" or a "net loss?"
If the extra hour simply adds one more portion of the same work she gets the other 4 days, I'd lean in the direction of not adding the day. I could, however, be easily swayed the other direction if the added hour was spent teaching the kids how to properly train and explaining how the strength portion of their practice time pays big dividends by making gymnastics safer, easier, and personally rewarding.
So look into what it is they'll be doing, ask the coach if your child was one of the kids they were thinking of when they came up with this idea, and then figure out if resting is more important than conditioning, because added strength work won't do her any good at all if she starts out each week feeling tired as if it were the last day of the week. Gymnastics, as many will tell you, is a "long haul" sport, so remember to plan wisely for the entire trip.