This trophy/achievement is exclusive to the Definitive Edition, and is similar to Till Death Do Us Part. As you defeat the bosses of the Drustan Trials, you will be able to make a wish. One of the wishes is to “I wish I could pick someone to stay with me forever and ever!” This option will allow you to pick Gemma or one of your party members, with an event taking place back in Cobblestone. Complete that little event to unlock the trophy/achievement.