Can an unlocked GSM that was used on AT&T work for Verizon?

So, today I was looking into buying a used phone from somebody. I get US Cellular and Verizon coverage and my area, but AT&T and other GSM carriers are basically obsolete. The phone I was looking at was unlocked, BUT he used it for AT&T and StraightTalk. I understand that AT&T uses GSM technology, which means the use of a SIM card. CDMA means you re digitality tethered to your provider until they unlock it, wherein with GSM, you can basically move between any GSM network provider by changing SIM cards.

However, if IIm correct, Verizon uses GSM technology for their LTE network. So technically, shouldn t the phone work in my area? I would appreciate some enlightenment in this area because IIm confused.

NOTE: the phone I looked at is an 4G LTE phone and IIm asking about this with the intent of using TracFone s Bring Your Own Phone program. I have an iPhone 4s with TracFone s BYOP right now.

Blah blah blah… If the phone is unlocked, you can use it on another GSM carrier.

Cell Phones & Plans

No. As I understand it, you can't bring a GSM or AT&T 4G/LTE unlocked phone to a Verizon network because Verizon does 4G/LTE differently than AT&T does.

If it is a 4G/LTE AT&T unlocked phone you should be able to bring it to Tracfone's 4G/LTE AT&T-based service. If it is an unlocked $G/LTE Verizon phone, you should be able to bring it to Tracfone's 4G/LTE Verizon-based service. Tracfone offers 4G/LTE for both.

If it is a 2G/3G unlocked phone it may need to be a Verizon-type CDMA phone.