The ship
Boy is she big! I knew she was 93,000 tons or so but until you see her & get aboard you don't start to feel it and understand the implications of sharing with up to 2300 other people. The thing that I was very aware as a single passenger was that meeting someone was no guarantee that you would see them again. There are so many eating and drinking venues that you are unlikely to get to know people unless you spend more time in one venue because the style suits you, then you will meet others doing the same.
The other thing is that unless you are pretty fit you will use the lifts (elevators) a lot. This is a horrible issue at embarkation & disembarkation time with only 4 lifts near the gangway. I tended to use the lifts when I went up several floors and the stairs to come down. The lifts are often full, particularly the amidships set and with children in buggies and wheelchairs also in use it could be quite a wait at times.
I suspect this is not unique to Gem but is a issue with large ships, as it is hard work to use the stairs to go up 4 to 8 decks to get between public areas.
It's very clear that public space is expected to pay and the only "quiet" areas I was aware of were quite small and out of the way on desk 12. This also meant that on the upper decks the loungers and tables had priority over movement - how you get to some I couldn't see ( maybe climb over others?). I found that 2 people going in opposite directions had to duck into gaps and powered wheelchairs needed a completely clear route. The series of pools and sundecks are pretty extensive but felt disjointed if you wanted to move around as I did in Amsterdam to capture the fire-float as you had to traverse various bits of deck and assorted ladders to get from the bow to the stern.

Food
Well I only managed to visit a few of the many venues (not enough meals or gluttony) but was generally pleased. The range of specialist options was wide and the Blue Lagoon was a good general venue as well as having a view of the 2 stories high giant screen. I had Wonton soup and Singapore Noodles which were excellent. Cagney's was well worth dressing up for and between the starter and main course I was full, so good portion sizes as well as really rich food. I found the cones and holders for serving the fries a little out of keeping with the rest of the style but not a problem.
I suspect that not everything will suit all tastes but the "queue-ometer" by restaurant entrances seems a good idea if an alternative is needed but I never really had a chance to see it in action.

Service
Well I have heard so much about the wonders of service on American ships maybe I expected too much but this seemed to be a problem. The bars were a nightmare at times and I had to sit out 3 or 4 tunes in Spinnakers "50s and 60s sok hop" while waiting for my drink and card to return so this was better that 10 minutes for a drink. This is less stressful with a group as the "waiter watch" can be rotated by the card holder will need to turn up to sign. I almost ordered 2 drinks at a time and was never remotely tempted to add a tip to the 15% service charge - and that grated sometimes. The restaurant staff were OK but I wouldn't rate them higher than elsewhere and there were occasional minor fumbles.
The team that DID have its act together was the entertainers in Spinnaker. I liked the dance offerings but as a single traveller often lack the confidence to make that first move, once I'm on the floor it's a different story, but the large team all invited people to join in and that included the ladies getting gentlemen out there (and I could get VERY out there!). The band was also great and although they were performing covers did very well and with lots of dancing and involvement from the singers.
I never saw my room stewards until I was disembarking and they checked if I had left the room. The basics were performed well but not exceptionally so.
Certainly not what I had hoped for.

Entertainment
I'll apologise now for missing the main show (I had food in my mouth at the time) but I was told the main show was good and the theatre looked spectacular. There was a good variety of bits and pieces in the different venues and the performances I saw were generally good. There were even options for the ballroom dance brigade, which I have recently joined, as well as the more usual rock and pop sessions.

Arriving and departing.
This is more the Port issue but the 2 things I disliked were having to carry my own baggage aboard and the pestering photographers. The luggage became a huge issue on board as few people were able to use the stairs. The crowd in front of the lifts was large and didn't seem to move, in the end I dragged my case up the stairs which was not an option for many. The photographers were a nuisance as the queue though security was slow and they were stuck in the way - a simple "No" was ignored and several passengers near me were really put out which is not a great start to a holiday. Later I ignored them instead.
The queues at departure were even worse and I seemed to spend 15 minutes in front of the lifts on deck 6. The single gangway is a real choke point and having 4 security desks where you could only get at 2 wasn't much help. There was also a free-for-all rather than calling groups and cabin numbers a few at a time.

General
Well the weather was spectacular and calm, the company brilliant from here and most of those I met and Amsterdam was itself. There were a couple of extras for a first trip, fireworks in Dover and a fire-tug in Amsterdam which added to the sense of occasion.

Summary
A spectacular ship in many ways but a "bit big" for my comfort zone. The Inaugural cruise was always going to be a bit of a "soak test" for the shipboard systems and staff and in general it succeeded. There were problem areas which I suspect are under investigation as we speak but the staff issues may be line style. I enjoyed it as much for the novelty as the trip but I would have to think hard before another unless it was with the same team. I think I may try another NCL cruise just to see what's different but my expectations won't be so high so it may go better.