Gaby Seyfert won the silver medal. She was coached by her mother, Jutta Müller, who would go on to coach 1980 Olympic champion, Anett Pötzsch, and 1984 & 1988 Olympic champion, Katarina Witt. After the 1968 Olympics, Seyfert went on to win the 1969 and 1970 World Championships, after which she retired. Had she not retired in 1970, she would almost certainly have been the favourite to win the 1972 Olympics. The reasons for her retirement are unclear, but were discussed recently in this thread
here. The answer may be contained in her autobiography which I recently obtained a copy of. Its in German and no English language version was ever released. Hence, as my German is very poor, it will be some time before I find a possible answer in there!