I was on the Gem 3 weeks ago and we did NOT embark / disembark right at the city pier. it was changed to Ormeggio 57 south of city center. NCL informed us of the change a couple of days before, but not that they would have a shuttle bus running from the “normal” pier to the new one. So we took the city bus and walked the last few hundred meters.
We got there right on time for the 9am slot marked on our cruise docs … but were told that the guards wouldn’t let us pass the gates until 10. We stood there for an hour, more and more people arriving by taxi, being told that they couldn’t go further. There were around 20 of us when finally someone from the port came, saw the situation and called a shuttle bus to take us for the last few meters. After that checkin was a breeze and I have never been on a ship earlier than this - I think we were up on the pool deck at 10:50!
If your pier is changed to Ormeggio 57, I would recommend walking to the city pier (route posted by ziggyuk) and taking the NCL shuttle bus. Or a taxi if you have an arrival slot after 10 am.