What happened was GSG abused the fudge out of their opponent's picks.
Stealing Shen, Ezreal, and Sona was their mistake, and they might have realized this before they grabbed Cho'Gath.
Shen's whole purpose is stalling to late game and winning. He's a safe lane that can jump in with a jungler's help, jump down to bot or mid for a 3- or 4-man gank. He's a late-game splitpusher.
But early? He's... nothing. He has low base attack, no AoE, and kinda crappy sustain.
Ezreal + Sona bot lane's whole purpose is to poke to death and farm hardcore. The sustain and poke makes laning against them ridiculous. So, what better way to negate them then to NEVER LANE AGAINST THEM. I mean, Ezreal's OP in a lot of ways. But, he has no AoE clearing other than his ult. Again, this is later. Not early.
GSG had this strat prepared for this situation, and were baiting out the picks with their own, while making it look like a plain old game. However, look at what they picked:
TF - Straight line clear/poke, stun (follow-up), mana sustain, global ult
Olaf - Straight line clear/poke, general OPness, really good against Shen, teleport
Caitlyn - Straight line clear/poke, longer AA range, traps, nets (zone control)
Blitzcrank - Pulls, pop-ups (zone control), extra tower damage with fists
Heimerdinger - Pushing power turrets, stuns, poke (zone control), passive health regen
Now, the strategy works with Olaf basically controlling both top and bot lane. And, with 4 people pushing mid, you need at least 3 people to control it. This has just removed the jungler from the game, but Ezreal and Shen can both solo level. This would work well, if Shen and Ez could push early. They can't. Sona can't help push either. Cho'Gath can with his AoE (you see him trying to clear creeps), but it's weak early, and since he's not jungling anymore, he's getting way less experience. This also allows Olaf to push really hard, because, hey, no jungler.
So, this powerlevels Olaf, giving him more experience than anyone on CJ Entus, and since Olaf is a god among mortals, he just runs through the enemy team, killing as many as he can. Notice that Olaf is NOT the tank, though. Blitzcrank is the tank. EVERY SINGLE FIGHT, he draws as much aggro as possible. In fact, he dies all the time. He's worth nothing to kill at the end, but makes the enemies worth money by dying to them. And they waste summoner spells to do it. Several of them.
This was a really good deconstruction of the meta by GSG, and allowed them to turn the game around by using early game pushing power to take towers and get a gold lead. Notice how Shen and Ezreal cannot push very fast early, but Caitlyn and TF are taking turns wiping out the waves with their straight line pushes. When Olaf come to help, he's doing the same thing.
Top this off with Blitzcrank making sure the three enemies in his lane cannot cs. If anyone gets too close, he'll grab them, followed by either TF stun and/or Heimer stun. Sure, he'll die to tower, but that's the point. He's TRYING to be the fall guy.
Now, as for items, they stock up hard on elixirs and Doran's. They need the health to compensate for lost levels, plus the other stats are boni. The only building seriously is Olaf, who is the only one actually laning. Caitlyn and TF are building somewhat real builds, but they're still outfarmed, so, that's just building what they can. Heimer and Blitz are just needing the health from the items, along with the AP and AD for pushing power.
Plus, one last point: Early inhibitors are destructive. And with no AoE clearing, how can you fight super minions? You CAN'T. Ezreal's not built to push. He's built to poke. Sure, he gets a minor AS boost for spamming skills, but it's not enough, especially early, to fight super minions. Plus, it removes him from protecting again. There it is again.
TL;DR: CJ Entus picked a winning combination... To a different game. GSG forced the enemy to play in a disadvantageous way.