Buying an iphone to use with straighttalk network

So I'm going to buy an iphone 4s and use a straighttalk unlimited plan. I'm not sure if it has to be gmsa or gms. Should it be originally from VeriZON OR ATT? I don't know much about it. Somebody please help before i go spend a bunch of money on a phone I can't use.

If you buy new and set it up at that time, then it will be working when you walk out, so you don't have to be concerned.

As long as the phone is iphone factory unlock, you will be able to use it to different carriers even straight talk.