Is there a difference between 'Gegenstand', 'Sache', 'Ding', and 'Dinge'? Can we use them interchangeably?
First of all, "Dinge" is simply the plural of "Ding".
Then, while all of them have a meaning referring to an object, there are very few instances where you can use all 3 terms interchangeable in the same sentence (I cannot even come up with one) and there are many uses for each of them where none or only one of the others would work, too.
In these 3 sentences, you could not translate the used term with either of the other 2 terms and keep the same meaning of the sentence:
- The subject matter of the discussion was a new movie: Der Gegenstand der Diskussion war ein neuer Film.
- This is a complex issue: Das ist eine komplizierte Sache
- This thing here is really complicated: Das Ding hier ist echt kompliziert!
Some more explanation for the 3:
Ding - Rather used in casual speech, addressing a tangible object
Sache - Referring to a thing, an affair, an issue
Gegenstand - The most technical term. Referring to an object, subject