In mine , the sims in farest possible seat (near window, back wall, in original lot) have had time to go there just fine. Its 7 squares away.
My custom building, farest seat is 9 squares away from stage, haven't noticed anything off there either, but my sims like to sit as front as possible.
You can test and visit with any sim, as gala is happening anyway, if your sims are nominated or not :)
hope this helps :)
ETA: I do use a mod by Bienchen , "sit at gala " , as in vanilla they used to indeed walk around or sit at floor :D