It seems like you want to say something like "The [other?] residents of the apartment complex" or "apartment building". Also, "out of humans" --> "empty" or "unoccupied" in this context. Nobody was there, right?

In english you would not use "retireds" like that, you need to say "retired people"

"The residents of our apartment building are mostly retired people."

Thanks, Adam. You were the best, but I couldn't choose your answer as the best one!

I think the meaning of what you are saying is better described as "The residents in our apartment building are mostly retirees (retired people), so the gym does not have many people in it in the morning.