From what I've heard, it's pronounced gwin-AY-ruh in some parts, which is my favorite pronunciation. This is a gorgeous, lilting, fairytale name fit for a princess. If only I were Welsh!
Actually since it has "gwyn" (which is masculine as opposed to the feminine "gwen") it would be pronounced "gwyn-IE-ra"; it would have to be spelled "Gweneira" for it to be pronounced "gwen-IE-ra".