The phone may be able to be used on both carriers, I don't honestly know, as my knowledge of the subject is several years old at this point. I do know that all major US carriers use the same-ish LTE bands in the GSM spectrum, so it would be cross compatible on that front, but 3G and voice, etc. may bring issues with it.
That being said, it should be noted that most phone manufacturers make (made if this information is actually outdated and most phones are cross-compatible now) 4+ different SKUs of the exact same device- one with Verizon CDMA bands, one that supported both AT&T and T-Mobile GSM bands, a Sprint CDMA model, and several international models which would typically run GSM.