Google Voice is not a voip service and cannot make internet calls. In Europe, the only way to make calls is via wifi using a voip service like Skype, Fring, etc.
Other options would be to get a cheap (used?) gsm phone and slap a prepaid sim in it when you get there for calls. They have cheap traveller's phones there too, I believe.
Verizon can also give you a loaner for travelling- probably charge an arm and a leg though.