Yes,all Verizon phones are GSM unlockedMy One M8 works fine here in the Netherlands on 3G/HSPA+, and so did my Lumia 928. 4G doesn't work though, but I knew that before I decided to get it. I don't think AT&T works here since they are CMDA which is American only (or at least not compactible with NL/GER since we basically use the same bands.
MMS works out of the box here and APN was pretty easy to fix in my case. Best of luck!