Götterdämmerung rewrites Germany from the foundations up — the biggest, most-requested tree in the game — and along the way makes the nuclear race something you actually play, not a background event.
What New Content It Adds
The pack splits its additions between the biggest tree in the game and two system-wide mechanics that give any game a long-term goal, wherever you play.
Nations: Germany and Central Europe
Germany gets a completely rewritten focus tree, with far more alternatives to the historical path. 
Austria, 
Hungary, Belgium and the Belgian Congo get content of their own with real weight for the first time.
Mechanics: Special Projects and Raids
Special projects are the big addition: a separate research tree for cutting-edge tech, from nuclear weapons and rockets to experimental tanks and planes well ahead of their time. It's the first time the game treats the nuclear race as something genuinely playable, and it changes the pacing of any long game that stretches into the mid-1940s. The raid system rounds it out with quick special operations against specific enemy targets, no large-scale offensive required.
Is It Worth It?
If your plan is to play Germany, this is the clearest purchase in the whole catalog — it's literally the rewritten tree of the most-played nation in the game. And special projects, being available to any country with enough tech, give a long-term goal to games that otherwise run out of objectives once the main war is won.