First, you have to know what GSM stands for.  GSM stand for the Global System for Mobile Communications.  It is the most popular type of cell phone network in the world.  T Moble GSM is the name and type of T Mobile’s network.  What you need to know is that the fact that T Mobile runs a GSM network means it’s easy to take your cell phone overseas and still use it.  Contact T Mobile if you’re planning on traveling first, they may need to flip a few switches on their end for your phone to work.