Sorry for the delayed response... (work wife gym)
I'm not exactly sure what your question is but I'll do the best I can to answer it.
GSM and CDMA are cellular technologies. Neither has anything to do with being locked or unlocked. Being on AT&T and having a GSM phone goes hand in hand as att is a GSM carrier. You can have an unlocked GSM phone on AT&T.
Example: my phone is a nexus5. It is a GSM phone that is unlocked and not carrier branded. When I first bought it I used it on t-mobile. Now I use it on AT&T. Because my phone is unbranded and unlocked I get updates straight from the manufacturer of the phone (Google /LG) and don't have to wait for att or any other carrier to approve the update. Also there is no carrier logo on my phone and no carrier software on it either.
Example 2: my wife's phone is a carrier branded locked galaxy s5. It too is a GSM phone. It has AT&T carrier software as well as all stuff from Samsung. Because her phone is carrier branded updates work differently for her than me. Google updates android then Samsung makes some changes to it. Then sends the update to all unbranded Samsung Galaxy s5 phones. BUT in the case of branded phones the carrier (AT&T in this case) takes the update and sometimes takes months to apply there own changes to it and then pushes the update to all carrier branded att gs5 phones.
Hope this helps.