You did not mention where in Europe you intend to go, but
my rental company of choice for Germany/Switzerland/Austria
is Sixt. They have a good selection of cars with GPS units.
Their premium cars (BMW 5-series, M-B E-class, etc)
come with GPS included. Standard cars can usually be
reserved with GPS units for an extra charge of about
6.00 Euro per day.
Slightly off-topic, note that you will save significantly
on your total rental cost if you avoid rentals at airports
or railroad stations in Germany, as the "facility charges"
are high.
For example, rather than renting at the airport counter
at FRA, I take a taxi (40.00 Euro) to the Sixt office in
Eschborn, about 15 minutes away, and save 20% or so
on the rental cost.
At the end of the rental, you can return the car to the
airport office at no extra cost.