There's no way to root the OTA Gingerbread update at the moment. The only way to get rooted Gingerbread is to install the update through the Team Black Hat app. So, if the OTA Gingerbread update has been installed, you can't flash a recovery of a previous backup because you won't have root access. I don't even think an update.zip from Froyo would work after you've upgraded to Gingerbread. A SBF flash might be the only way to downgrade to Froyo from Gingerbread.