Shouldn't it be "was" instead?

yes

Thanks.

Shouldn't it be "when" instead? (of "where")

The “is” is correct.

The meaning is:

One shooter is not enough. Gone are those days. Today we want...

As Region pointed out, it should be "was." As usual, Punkey is wrong. As to the use of "where," that is acceptable, at least in American usage.

I said “is” was correct - not more or less correct than “was.”

In fact there are many verb tenses that could be used in that adverbial phrase, depending on context and time- which the OP failed to give.

Gone are the days when one person’s answer is exclusively correct to an ambiguous question.

BTW, I think I have acknowledged when I was wrong made in any post. That’s not always been the case from others.