Judging by what I've found so far, it can mean either. Is παντρεία a more suitable word for marriage while γάμος is still somewhat acceptable to most people, kind of like generally "marriage" and "wedding" are sometimes used interchangeably despite having different meanings?
You are correct about the "It can mean either" part. In Greek, we don't have different words for marriage and wedding, it's γάμος for both. Παντρειά is way less commonly used, and it can mean only marriage. I wouldn't suggest using it. ^.^