If you bring a US GPS, there may not be maps of Europe. I have a $400 Garmin, and I had to pay $100 to get the European maps, and they still did not work at all - it told me to drive into rivers, and was a complete disaster. I think it kinda worked 2 days out of a 9 day trip, where as Google sometimes worked better WITHOUT A SIM card. And Garmin don't even allow you to transfer those expensive maps to another Garmin device. I am selling it if anyone is interested.
On the other hand, I have had the best luck getting a European SIM card via Amazon from Modal in the UK for $50-60, and use Google - you can buy that in advance in the US.
Slightly worse was a French Orange SIM card, about same expense, but ran out of data very quickly (this was in 2014) and reception not that good. In my case, I arrived in France and signed up for the card, which took 40 minutes as I recall. I remember walking around the Opera area of Paris and not getting any signal at all. So I wouldn't do it again.
Warning - all SIM cards are not the same. I bought some cheaper UK SIM cards on Amazon - like Lime Card I think - and they did not explain the fine print that it works in EU but at $2 a minute, and I tried to call their customer support in London, and got recordings that used up all of my limited time. I thus got ZERO value from the card, and I came back to the US, and Amazon would not allow a review of the device.
I also used a Huawei hotspot/GPS that came with a French car rental. I think it cost $10 a day, or $300 for a month - which is comparable to Verizon's charge for a foreign plan. The signal worked pretty well, and I was able to use my US phone and google. The map on the Huawei was a bit substandard though.