Verizon has been releasing phones with GSM radios for a while. Being a CDMA carrier, their hardware prevented people from traveling with their phone outside of the US. So Verizon started including GSM hardware in their devices, often locked away. They eventually updated the phones to unlock the GSM radios for international travels.
The HTC DROID DNA is a different story. Apparently, it comes with its GSM hardware fully unlocked. With a new AT&T or T-Mobile SIM card, it will work perfectly. It supports 850/900/1900/2100 MHz bands, so it’ll work on any carrier in the world with those bands. All you need to do is change the APN. If you wanted to, you could buy one of these off contract and use it with a different carrier. Just beware of updates, Verizon has been known to further lock GSM radios with updates.