Dear @Nay,
Thank you for your question and for being such an active member of the community!
I've tried to find a source that names or shows the countries that a "Grey passport" holder (in German: Reiseausweis für Ausländer und Staatenlose) can travel to and couldn't really find any.
According to the residence ordinance § 9, holders of a "Reiseausweis für Ausländer" are not allowed to/ should not travel to their country of origin ("Herkunftsland"), as you would lose your right to stay in the EU-country.
The paragraph states that
"The Travel Document for Foreigners can be issued for all states, or with a restriction of the territorial scope to certain states or parts of the Earth. The state of origin that the foreigner holds citizenship of is to be excluded from the territorial scope, if an extension of the scope to the country of origin isn't justified in exceptional cases." (I just translated this, so it's not perfect)
(1) Der Reiseausweis für Ausländer kann für alle Staaten oder mit einer Beschränkung des Geltungsbereichs auf bestimmte Staaten oder Erdteile ausgestellt werden. Der Staat, dessen Staatsangehörigkeit der Ausländer besitzt, ist aus dem Geltungsbereich auszunehmen, wenn nicht in Ausnahmefällen die Erstreckung des Geltungsbereichs auf diesen Staat gerechtfertigt ist.
As I read in another source, the responsible "Ausländerbehörde" (foreigners authority) can decide the territorial scope of the travel document. Therefore, it might be best to ask the responsible authority that issued the document which countries you are allowed to travel to.
This is not a complete answer of your question, but I hope it helps a bit! If I do find any other information, I will let you know.
All the best,
Isa