That awkward moment when...
...you get the rules polished up nicely only to realize that in the process you created a sound change (or, in this case, a lack thereof) which breaks your language’s name.
More specifically, there’s a rule that very explicitly states that only /j/ and /ī/ - not /i/ - cause i-umlaut. Therefore, “Gutish,” not “Gytish.”
In other news, the new complete rules coming up momentarily.