SIM card to use in both France and Italy?

Roaming is free in the EU. The SIM you get in France will work in Italy. We had one from Bouygues Tel
and experienced excellent service and connectivity. Wherever you decide to get a SIM be certain it works when you leave the store to avoid having to go back,

We'll be arriving in Italy first so I assume that the reverse is true... the card we get in Italy will work in France?
Thanks!

EU roaming without fees is not unconditional. There are lots of exceptions. But, generally, yes, you should be able to buy a prepaid SIM card in Italy that will work there and in France. Just be clear when buying it in Italy that you need to use it in France, too.

Also, make sure you learn how to set the APN (Access Point Name) - occasionally, you need to set this (maybe just on Android phones) to use data. It's an easy thing to set in your phone's settings, once you know what to look for, but it's not intuitive to set if you don't know what it is. Samsung phones seem most likely to need to have this done. It may also work in Italy without setting the APN but stop working when you get to France - but change the APN to the correct setting and it may work fine.

And by the way: you don't actually need mobile data or even a SIM card to use your phone as a GPS, believe it or not. GPS itself works without any mobile data connection. But...the maps themselves must be downloaded ahead of time if you have no mobile data. Google Maps for example has an "offline" mode that lets you download a map area ahead of time, so you can drive with your phone in airplane mode and still navigate with your phone as a GPS. It's still nice to have data (offline, you can't get traffic info) but you can keep it in airplane mode if you like to save data if you want. Just set the offline area ahead of time and download the map then.

Offline maps only work (in Google Maps, anyway) for driving but not for walking or public transit.

Thanks Laurel and Andrew! Your comments were very helpful!