The above posts miss an important point: when you rent a car, there's always some degree of random-ness involved. You may, or may not, end up with a car with a built-in GPS (just like if you request a car with automatic transmission, the car you get may or may not have one - you get a car they have, and they may not have exactly what you requested). The degree of randomness varies with location and company and who-knows-what-else, but there are no guarantees you will get exactly what you ask for. Just keep that in mind.
It it were me, I'd bring your Garmin (I always bring mine). Worst case, maybe you end up with two GPSes - which I actually find useful at times (compare/contrast what they say - sometimes they differ quite a bit, which is a good reminder that YOU are the actual driver and navigator with skin in the game, and you shouldn't just blindly follow everything some gizmo tells you).
Personally, I think the best plan is to bring your own GPS from home (with current European coverage - make sure you have that before you start packing). Your own GPS is always in a language you speak, you know how it works, and you are familiar with its operation. When you jump in an unfamiliar rented car in a foreign country, you are already challenged enough (signs are in a foreign language, the driving style is different, etc.). That's stressful (and dangerous) enough for me already. The last thing I need is to have to try and figure out a new GPS that's bleating at me as I struggle to find my way and avoid an accident.
The first few terrifying minutes of driving overseas is very dangerous and you always need to be very, very careful at that vulnerable time. I have a clear memory of sitting in a rental car office in the UK, watching the American couple ahead of us confidently and happily get in their rental car, drive off the lot, and have a very loud, obvious, major crash in their first 100 meters on the road (the employee at the counter shot me quite a look - "don't worry, I won't do that!" I said).
Also: Never, ever rely 100% on anyone's GPS (or phone or tablet). Devices fail, run out of juice, get stolen or lost (all of these things have happened to me). Always, always bring along from home (where it's easier to find) a good paper map to use along with your GPS or other device.
Hope that helps. Have a great trip!