Hello I have an iphone 4s for sprint too and use gpp sim to unlock it. I already tried updating my phone to ios 7 beta and tried using the gpp sim to "unlock" the phone but I didnt have any luck. tried multiple ways and nothing really worked.
To recap I (restore option)installed fresh install of ios 7 beta, I paid to have my UUID registered. ( you dont need to do this you can update directly from ios 6 without that, I done it just because I rely on my phone tooooooo much and I didnt want anything to ****** it up). registered as a new iphone and nothing, no signal or anything.
Second time I updated directly from ios 6.1.3 (update) option. I registered the phone as a new iphone and still nothing.
By the way I am not Jailbroken. I lost my jailbreak when I accidently updated to IOS 6.1.3
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I done that already about the same time the rumors about the GPP sim unlocking ios 7 came around. They still havent replied to any of my messages