You probably won't see a gain (it's possible but not too likely). Even if you did, it would only be water retention and the week after you would probably have a bigger loss than usual. Obviously, as with everything, everyone is different, but that's what usually happens. It's possible to put on 5-6lb a week or so before your period, but then you lose it again during. So whenever your first weigh-in is after it's finished will show you what weight you really are. I know it might feel demotivational seeing a smaller loss, but try and remember it's only water, not fat and is temporary.