I bought an iPhone 7 from AT&T, and the device is not compatible across the major carriers.
You're not using the term 'GSM' correctly. You mean to say that your device is locked to AT&T, not that it is 'GSM'-only.
Actually, you’re wrong. Att sells an iPhone that is not compatible with CDMA networks. Trust me bud, I’ve done all the research. Apple sells their devices with the ability to use any of the 4 carriers.