GDD is the abbreviation for game design document. The GDD is written and maintained by the game designer. It describes all game features in detail and is the go-to document for the development team. Sometimes it is also used as the basis for a publisher contract because it states all features that will be implemented.
GDDs used to be the single source of information while developing a game, but this changed in recent years. It is difficult to handle multiple GDD versions/to ensure that nobody works based on outdated information. Multiple game designers can’t work on the same document and it is hard for the programmers to find the important implementation details. The trend goes to online hosted wikis.
At the company, we use mainly wikis hosted on confluence. These wikis are maintained by our 2 game designers and are sometimes made available for our QA testers. Sometimes a GDD is still needed to apply for funding or something similar. For example, the FFF Bayern requires a GDD as part of their evaluation process. As an example, I will link you one GDD I wrote for a small game in university.
Hope you can understand german: