you need to have ONE of the chapters ---that means either prophecy, factions or nightfall.....so if you have factions you will still be able to use gwen. If you have nightfall you will be able to use gwen...
It only requires you to have one of the stand alone chapters to use the expansion. If you have more chapters you will still be able to use gwen, it doesnt matter which one, as long as you have one.