[gumby]

It doesn’t undergo the phonetic process of umlaut, Don, and as it’s a vowel the two dots are more like a simple diaeresis.

It is used in that way in French too.

[gpvos]

It is neither umlaut nor diaeresis, but just an i and a j with each their own dot smushed together into a ligature, which can take either the form of a y with both those dots on top, or a more rounded form that more resembles an i and j joined together.

I guess in other languages than Dutch it could be a y with a diaeresis or umlaut.

[DonHopkins]

Verbal diaeresis?