For starters, the radio stations would be replaced by the all-seeing, all-knowing DJ Atomica, the music would be considerably less eclectic, and the ZiT in-game feature that lets you know what song you're listening to would simply be replaced with actual text pop-ins, the way every other EA game does.
After all, your cellphone would actually be an iPhone, or a Verizon LG8600, or a RAZR.
The GTA games are known for, among other things, their incredible attention to detail when it comes to satire, and this goes all the way down to the fake commercials on their radio stations, not to mention the tons of signage and branding for their fictional properties - BurgerShot, Sprunk, Cluckin' Bell, etc. EA, on the other hand, is known for their incredible devotion for gratuitous in-game advertising; it would surprise absolutely no-one to see an EA-published GTA take place in a world with real commercials and real billboards for real products, which you could then buy in-game. Imagine getting a health power-up from eating a Big Mac!
Let's see... we've got the EA Trax soundtrack, we've got the gratuitous in-game advertising... well, I suppose we can extend that to Branded Achievements. Drive for 1000 miles and get the GoodYear Tires Achievement; crash into 200 cars for the AllState Insurance Achievement; snipe 100 headshots and get the Johnson & Johnson "No More Tears" Achievement.
Of course, it would be hard to imagine any wholesome product that wouldn't be attacked from the Jack Thompson side of the fence if it allowed itself to be placed in a game this subversive, notorious and controversial. (It hasn't stopped UbiSoft from putting car commercials in Rainbow Six games or Nextel devices in Splinter Cell games - or car commercials in Crackdown, for that matter, but let's not get ahead of ourselves.) This is a long way of saying that GTA5 can't feature any more killing of hookers for points. (Which doesn't happen in the actual GTA, but I digress, again.)
So the storyline and cast of characters would have to be switched up. Which would mean that instead of featuring a compelling and epic narrative about the troubles of a European immigrant fresh from the horrors of war, we'd get a 10-15 hour story about Bobby Flash, a streetwise white kid from the suburbs who hopes to be a skateboard champion but who runs into a gang from the other side of the tracks, probably featuring tons of ridiculously over-the-top FMV cutscenes that likewise fill up the Need For Speed franchise - hey, maybe they can throw in a Need For Speed billboard or two, while they're at it!