Thread: Gone are the days when ...

This is just "inversion" used for extra emphasis. As in:

Of that I have no doubt! Her I like! Never should I have done that! Never did I know what would happen etc, etc.

Your original sentence should just be viewed as:

The days are gone when...

Nothing more, just extra empasis on gone with that position, most likely to express sorrrow or regret for the good old days.

E.g If you watched the first "hunger games", there is a moment when Catniss gets interviewed before the games. IIRC in the original she says something like: "I will try to survive" and the interviewer response was:

"And try you will". This clearly expresses doubt that she will succeed but acknowledges that she will indeed try.