Free Moto X
Verizon Wireless is the only major wireless carrier today that does not put a software lock on any of its 4G LTE smartphones. This means that whether you are on a contract or you paid full price for your Verizon 4G LTE handset, it is automatically unlocked.
You should be able to put a SIM card in it from any GSM carrier and it will offer basic voice, texting and 3G data. Whether it will also support LTE depends on what LTE frequency bands are
supported in the phone.
A Moto X designed for Verizon will work with either an AT&T or T-Mobile SIM on the gsm network. But you may be limited in terms of the service that you can access. As I said, you will definitely be able to text, call and use 3g, access to LTE will depend on the frequency band being used in your area, but most likely you'll be covered.
As far as the nexus is concerned, it supports LTE Band 4 AWS, which is also supported by AT&T and T-Mob on GSM, although not everywhere. A simple, cheap prepaid SIM from one of these networks popped into your nexus will confirm if it will work.
Hope I was able to help out, so now what are you waiting for? Go get yourself an AT&T or T-Mobile prepaid SIM and free your Gnex.