It should be: " The/That era has gone when people used to attend political rallies. " "The" and "That" are both possible. There is hardly any difference in meaning.
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Emanuel Guerrahowever this/that has a location/distance meaning, it also relates to something that has been said previouslyYes, that's a good point actually -- one that I overlooked. "That" may be more suitable when referring back to a previously mentioned era. Because of the nature of thes sentence, I think I was assuming it was the first mention of the era.