This happened to my friends Guinea pig, once one died, the other became depressed and around a month later he died. Not wanting to worry you here but I would recommend getting another gerbil to keep him company and raise his spirits.
My gerbil nashoba just passed away last week. I think my gerbil may be depressed. He is sitting in his little cot sleeping all day and rarely will be come out he only comes out for food I also think he may be dying. Someone please help meš¢?