What is it. What does it mean? What does it do? Is it better or worse? Heeeelp
The Google is your friend:
What that means for your Sonata is that the valves could build up carbon sooner than the regular engines. You have to be more diligent using top tier gasoline and doing regular oil changes. I have been using synthetic just to help. Some install a catch can but I have decided I don’t want to mess with what the engineers have designed, hoping they have some clue what they are doing 
It’s proven to have problems.
The theory is that by spraying the fuel directly into the cylinder rather than into the intake port turbulence of the fuel from its having to get around the valves is eliminated, creating a more misty mist. With gasoline aeration, that helps in theory. The finer the droplets, the more surface area there is per volume of fuel used, and it’s only the surfaces of the droplets that burn. Gasoline droplets burn in layers like peeling an onion. A finer mist means the fuel burmn more quickly and completely.
The trouble as I understand it is that the gasoline with its detergent additives also constantly washes the backs of the valves and the valve seats as it moves past them. If the fuel is sprayed directly into the cylinders, that action doesn’t happen. Without that action, carbon builds on the intake valves.
I’d avoid it until the bugs are worked out.
the gasoline is directly injected into the cylinder, avoiding the intake valves altogether. The intake valves are used for air only. They do inject gases from the exhaust gas recirculation (EGR) system. The EGR gases contain unburned or incompletely burned fuel. This fuel hits the hot valves and burns on the intake side, causing the carbon deposits. Some manufacturers have better success than others. But you own the Sonata, so knowing this can’t help you buy a car now. I don’t know whether your car’s engine is prone to carbon build-up on the valves.
Direct injection is how diesel fuel injected engines work. GDI is the gasoline-engine version of that. There’s plusses and minuses, its a compromise.
If PKA, SAE, LK and DKE don’t rush you, no worry, there’s more fun at GDI.
From what I have read, each manufacturer has their solution to the GDI carbon build up. Some have an extra small injector next to the valve. From what I have heard, Hyundai/Kia does something with the valve timing to stay open longer on start up and this helps. You will have to google and find the article.
As I said, this is one of the stuff that you will have to deal with when/if happens.
Many are going DI route, including Mazda, Honda, Ford, Chevy.
Toyota apparently long resisted going the GDI route, but I’m not sure what they’re up to in the last 2-3 years
Why do I see diesels with egr delete kits?
Toyota may have found the solution. The FRS engine uses a combination of direct injection AND port injection.
Yup… Only other way I could see to accomplish the same thing is Alcohol or Water injection where the vehicle doesn’t already have two separate fueling methodologies present… (a not so insignificant detail or expense) If I wound up with an orphaned GDi vehicle suffering from carbon…that’s what I would do…Water injection…except nowadays I would use a fuel injector and proper pressure pump to atomize the charge correctly…they sell complete systems out there I’m sure…people just don’t know enough or have reasons to employ them generally.
It would make for an interesting experiment.
Blackbird
…but Toyota doesn’t play games in the engineering department.
They leave that to the Lexus division. Lexus has been using direct injection in the rear wheel drive platforms since 2006.
Toyota fist used direct injection in the 2003 Toyota Crown sold in Japan.
The new “Italian tune-up” for GDI engines:
…when he traded it in at 70,000 the dealership said his valves looked almost brand new.
I’m sure the sales department performed a real thorough walk around when he traded in that car. That video is all the proof I need, well documented and supported.
Lexus has spent millions attempting to correct the 4GR-FSE carbon built-up problem, one day they might discover you can just drive it off.