The surname Gjestvang could be translated to guest-house, since 'gjest' means guest and 'vang' is a house or shelter. The proper pronounciation would be "yes-tvaang" pronounced in one word.
Most Gjestvang families originate in Hedmark county in Norway, and are found on either side of Lake Mjøsa. My own family originated in Rælingen north-east of Oslo. Their name was originally Gjestang. When my great-great grandfather moved to Oslo, then known as Christiania, around the 1880s, the name was mis-spelled by the name registry. For the last 5 generations we have therefore had the name Gjestvang. I don't know of any relatives who emigrated to America. It is therefore most likely that any American Gjestvangs have forefathers who came from Hedmark.