I´m not going to go back any further than the middle ages in this discussion. Old Gutnish has of course a longer history than that, but we will start with the language represented in the Gutnish Law from around 1300.
From this time the main source is this law, and runic writings in general and the runic calendar from 1358 specifically.
The main characteristics of the language from this time is that the old diphtongs are kept as ai, au and oy while it has already began to change in the other scandinavian languages. A special form in Gutnish is also the triphtong iau which derives from iu which on the other hand is kept in many of the other scandinavian languages and dialects.
Unique in Gutnish is also the word for ´she´ which is pronounced [ha:n] in Old Gutnish. In the medieval law it is spelled either han or haan.
More about this later.