Is there a difference between 'Gegenstand', 'Sache', 'Ding', and 'Dinge'? Can we use them interchangeably?
First of all, the singular versions of "Dinge" is "Ding".
Then, while all of them have a meaning referring to an object, there is a very few usage overlap where you can use all of the 3 interchangeable in the same sentence for the same occasion (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:
Some more explanation for the 3: