What firmware to download for iPhone 4 on AT&T?

I have this iphone 4 with at&t and i want to download a specific firmware for it and i don't know whether to get the GSM version or the CDMA version or what version to get can anyone help me find the right version

Download gsm and I have the same phone: D

AT&T should be a gsm model. Just check if there's a simcard and access panel on the right side of the phone, if it has then its confirmed a GSM one