If the phone you are looking at is ATT branded, everything will just work. Even still locked to ATT your GoPhone SIM is a go.
If the phone you are looking at is T-Mo branded, it needs to be unlocked to work with your GoPhone SIM. If it is unlocked from T-Mo your GoPhone SIM is a go.
If the phone you are looking at is a Factory Unlocked International it would be good to know if it was placed in Canada or Latin America as opposed to Europe.
Do you have any more detail on the phone?
I understand not wanting to share a link if you are worried someone will snap up a great deal before you can decide... If you like, you could PM me a link and I'll look at it for you. I have too many phones and don't need to steal your deal. LOLz