you should be able to flash gingerbread right over liberty,flash it in clockwork recovery just like any other rom, wipe data and cache, mount system and data and enjoy. just remember you cant flash a backup of froyo on a gb rom, you have to sbf to froyo
In clockwork recovery there is a option that say mount, you would go into that and mount system and data. Lot of these new roms do it for you when you flash so nobody forget to do it. So you might not have to with gb, I also ways do it to be safe.