Regarding the Gevey Sim, Buy at applenberry.com. and make sure that the version  as well as the firmware in the Iphone match the Gevey sim description. as far as i know , the website have only one type of gevey sim  for iphone 4 and two different kind of gevey sim for 4s.

make sure the one with GSM is for the At&t network. i mean if the iphone is unlocked by At&t then u need to use the gevey sim with GSM and another Gevey sim is CDMA type. the CDMA is if the iphone is unlocked either with verison or sprint, then u will have to use CDMA.

and if you have iphone4 check the version and the firmware going to the setting>general and about.... u will see the description of the iphone and check what is the version and the firmware of the iphone, as far as i know, for 4 s the gevey sim in the website  do not work for the iphone 4 which have firmware 4.12.01. so make sure the version and the firmware match according to the description of the gevey sim.

all the best
thanks