Will unlocked Dell Streak 7 work on any GSM Network?

I have a Dell Streak 7 that I received from T-Mobile. It works fine but I decided that I want to sell it. I contacted T-Mobile technical support and requsted an unlock code. Shortly after this I received an email from Dell requesting my IMEI #, service tag, etc. (which I sent them in a reply). About 6 hours later I received another email from Dell with the unlock code.

So I restored the tablet to factory state and then inserted my friends AT&T sim card into the device. When I turned it on it asked me to enter the code. I entered the code and received the message, "unlock successful". I then listed the item for sale on eBay. I just received a question from a potential buyer who wants to know if my tablet will work on any GSM Network. I am unsure what a GSM Network is and so don't know how to respond.

Will my unlocked Dell Streak 7 work on any GSM Network?

Thank you in advance for your help with this.

Bob C.