Your gearset to start off with is admirable that is for sure. You do a nice job of maintaining your 540 defense rating and definitely have enough gear to do some quality tanking in Naxxramas. I am not sure who told you that you should gem for expertise but I would very much disagree with them. You see, in your current level of gear, with the content that you will likely be clearing with it, threat is not going to be your largest issue. Expertise plays a role in defense by reducing the chance that a boss will get a parry hasted swing on you and hurt you. Mostly however, expertise is a threat stat because it acts sort of like hit. Reduces your chance to be dodged and parried by the boss.
What you mostly want to shoot for are a few bare minimums for tanking. You need to be uncritable (which you are). You need to be able to hold threat (which unless you are trying to tank for people who significantly outgear you, you can). And you need to make sure you have enough survival stats to tank all the bosses that you are expected to tank.
This is where the disagreement with your current gear setup lies. By geming for expertise you are adding threat stats when you should really be adding survival stats. If you were to place stamina gems in your various different sockets then you would be better able to take hits from a great many bosses in the raid instances. The higher the healthpool you have, the easier it is for healers to keep you up. As you progress throughout the raid instances you will find that bosses continue to hit harder and harder and that having as much effective health as possible will go a long way for you.
I would recommend not gem'ing for anything other than stamina except for when you are trying to match beneficial socket bonuses. By beneficial bonuses I mostly mean ones with stamina or perhaps a great deal of defense rating (that might get you over the 540 cap as an example). Overall your largest challenge when tanking in raids is going to be able to survive. Naxxramas is a very easy and basic instance but when you are first entering the instance, this survival is still something you are going to be worried about. As you progress through Naxx you will pick up gear that will give you very generous amounts of survival stats naturally and give you some room to adjust your gear to be more threat oriented. I still however, would not recommend gem'ing for much more then survival unless you were actually trying to build a specific gearset (like one that capped expertise).
Hope this helps.