I know little about the business of high-tech digital gear, but I can guess GPS makers such as Garmin and Tom-Tom are having a rough go. Increasingly, people use smartphones for guidance when on the move. For example, I no longer rely on my car's factory-installed GPS. iPhone's Google Maps app is easier to program, has much better maps, better guidance, and the destination can be changed by my wife while I'm driving (our car has to be stopped in order to request a new destination on the built-in GPS).
Depending on what it will cost for data while traveling in Europe, you might want to rely on your smartphone's Google Maps app. Another option is to buy a cheap, used GPS.
Whatever you decide, you will want to think long and hard about renting a GPS from a car rental company. They are expensive and don't always work. Rental fees range in price from about €5 to €21 per day. Some max out at €90 to €200 for a longer rental. And what you get isn't always the greatest. Rental devices are heavily used and we hear complaints they sometimes don't work properly. Most are multilingual but many renters find they are unable to set them to speak English. To rent a car whose price includes GPS, you will probably have to book more expensive fullsize or higher category cars. Few economy, compact or intermediate cars come with built-in GPS, and seldom is the price include in the overall rate.
The only problem with using Google Maps on a smartphone in Europe is the cost of international roaming. However, you can probably purchase an overseas data plan that will allow you to operate Google Maps while driving (prior to a recent European trip I paid AT&T $59 for a 300MB overseas data plan which, by being judicious, I didn't use half of).
However, even if you disable data roaming entirely, you can still use Google Maps in a limited way. Before starting out on an auto trip, connect the phone to the Internet via WiFi at your hotel or a café. Next, set your departure and destination points and Google Maps will create the route. Once you are in your rental car, even though Google Maps won't have access to the Internet, it will continue to show your position along the route by using satellites and cell towers. It won't provide verbal directions and you can't change the route, but it will keep track of your progress.
Another alternative is buy a used portable GPS. Right now on eBay, you can purchase used devices that include Europe maps for as low as $40. A Garmin Nuvi 670 with Europe and North American maps, which a few years ago sold new for over $600 (I know, I bought one) has a bid price of $49.99 and 'buy now' price of $99.95. My Garmin 670 still works great but it sits in a drawer; Google maps on the iPhone is better and a lot handier.