As in all languages there is a difference between what you see in writing and what you say. It´s just that the old written languages of the world is now fully accepted and taken for granted. So it is not with most lesser used languages.
The Gutnish language is old. No doubt about it. But for the last 400 years it has only been a spoken language. Well, with some exceptions of course. From the 17th century we have a few wordlists, in the 18th century the same, but also sentences and a couple of “grammaticas” and a few poems. From the 19th century we have the large work and collection of the two brothers Säve who spent decades in pursuit of collecting as much as they could about the Gutnish language and culture. From this time we also have a more scientific litterature concerning Gutnish. Also at this time newspapers started to be printed in Gotland and even though they were in Swedish, they sometimes printed articles on the language or about the language.