These are some of the APN setting that will work with Green Mobile for the GSM handsets.
Name>anything you want

APN> att.mvno

proxy>not set [blank]

port>not set [blank]

username>not set [blank]

password>not set [blank]

mmsc>not set [blank]

mms proxy>not set [blank]

mms port>not set [blank]

mcc 310

mnc 410

auth type pap or chap

apn type default
This is confirmed on most all phones although it does not get lte yet. I will update here with more to come including for apple iPhones.