So I looked it up and I have to update my statement before.

First, one can also just have their gender entry outright deleted.

Secondly, naming conventions are the same as for baby names:

Generally, first names have to correspond to the new gender entry:

1. Who chooses the male gender can use a name or names that are purely male ore both male and female.
2. Who chooses the female gender can use a name or names that are purely female or both female and male.
3. And who chooses the gender „diverse“or to delete their gender entry has free choice among male and female names
4. For all genders, the chosen names have to be actual first names.

(Source translation by me because I was too stupid to realize there's an English version)

Unfortunately, despite the actual progress this law certainly demonstrates, it also shows there's still both a binary way to think about gender and certain expectations linked to gender well seated in the minds of lawmakers. Nonetheless, it's a lot better than what we had before.