This sounds like you've seen some higher age GBs in other peoples cities. It is possible to have them if two conditions are met: they have all the blueprints for the building AND they were able to get hold of the right goods to pay for it. In many guilds, there are people in higher eras who are willing to do trades to help another player to get a certain GB (especially the arc), making getting these GBs more achievable for people in lower eras.