I am considering using my iPhone 6 as a GPS when traveling in Italy. No doubt I will also need connection to their mobile phone services. How does that work?

Depends on what exactly you want to do.

Your local cell carrier is sure to have Roaming plans you can use in Italy. That is you connect to italian cell towers, but you use your own cell service plan from home, and its charged to you current bill. Contact your carrier for information.

Alternatively if your iPhone is unlocked, you can potentially buy a pay as you go sim card form an Italian carrier and insert it into the iPhone and use a local Italian carrier instead.

Most of the time this can be cheaper than Roaming plans since you can control the Pay as You Go option. Though you may need to be able to read soe Italian to recharge the sim card when it runs out.

Well, the first thing you must do is have your device unlocked from your wireless carrier. They are the only ones that can unlock your device. Apple will not have any control in this. Just call them and let them know you will be traveling abroad and they can hook you up. Once you are there in the country you will traveling to, you can obtain service with any carrier in that area.

Once you have service, you will then have to restore the device with the SIM card from that carrier. Make you have a backup before you restore, as you do not want to lose your data.

You only need a connection to download mapping as you move around. If you buy a satnav app which includes the mapping within the app, you have no need for an internet conection. An example is TomTom Western Europe. Note that this makes for a big app. The mentioned TomTom approaches 2GB of storage.