Is it "needed"? Nah.

But does it help? Yes.

I've seen gameplay videos where even the 1080ti can't "max" out a game at that resolution and occasionally dip. And that's where gsync should help.

So the real question is... is it worth it to you? Check out YouTube videos to see people with similar hardware and see how gameplay is.

Edit: after moving my game drive from my old PC to my new PC, I tested a game. With an i9-7900X and EVGA 1080 ti SC2 Hybrid, at 2560x1440 (was temporarily using another monitor while assembling the PC) I had to turn the graphics setting in Ghost Recon:Wildlands down to just high for it to break 100 fps in the built in benchmark (around 110). That means even at high, it won't be able to keep up the max refresh rate on the ultra wide monitor. I set my max speed to 90 Hz, so I can pretty much get away with very high, since it hovers in the upper 80s, and let gsync deal with if/when the fps is much lower and keep things relatively smooth.

At ultra setting, it hovered at 60 fps, meaning it'll be even less on UW.

I will NEVER EVER use another Nvidia gpu with out a G-Sync monitor. This is my first monitor of that type and as you can see. I am extremely pleased with G-Sync. It is game changing and a must once you experience it. 

To expand on my earlier post (edited part)... I've finally moved my new PC to pair with my ASUS PG348Q that I normally use... So, while the framerate is lower than my earlier test (which was on a Dell P2415Q 4K but @2560x1440 for the test), the screens are much smoother across the board... despite being 3440x1440. On the Dell, you could see tearing when the camera pans across the village early on, which you don't see on the ASUS.
 
TL;DR
Dell: Lower resolution, higher framerate, no gsync, with screen tearing.
 
ASUS: Higher resolution, lower framerate, w/gsync, no screen tearing.
 
I didn't mention micro-stutter cause thats one area I don't always notice unless they're particularly bad... at which point, it's not really micro anymore.

No. It's not worth paying 200$ or 300$ more than a regular monitor. The only situation you have to get an Nvidia-sync monitor is if you are a professional gamer and you need milimetric accuracy. If you are just a normal gamer and play adventure games like witcher3 etc you don't need it.

Opolis

EdhuCesar
No. It's not worth paying 200$ or 300$ more than a regular monitor. The only situation you have to get an Nvidia-sync monitor is if you are a professional gamer and you need milimetric accuracy. If you are just a normal gamer and play adventure games like witcher3 etc you don't need it.

Or if you play games with fast movement such as sim racing and want a better overall experience.
 
I agree with most of you above.
Is it needed? No.
Is it worthwhile if you've got the cash? Yes.
 
The biggest difference for me is that when my FPS is lower, it keeps things looking smooth and I don't notice it as much.
I went from a 60hz display to 144hz Gsync and really enjoy it.
 

Yep. Only if your bank account allows you. Otherwise, is not a must. 
Better save those extra 200$ or 300$ and get a better GPU/CPU.

Jbj5000
I do quite a lot of sim racing, however i have not had a problem winning races on my 1440p 60hz monitor with a 1070. Thank all of you I will be going with the Samsung cf791

Nice. You do know Free Sync doesn't work on nVidia cards?

Jbj5000
I am planning on purchasing a 1080ti SC2 along with a 3440 x 1440 100Hz monitor, I have noticed that monitors with G-sync run $200-$300 more expensive. Will there be any noticeable impact on gameplay without it?

Once you go G-sync, it's hard to go back.

Personally, when I first read about it, it seemed interesting but more than likely a gimmick.  Then a few years ago, I worked Blizzcon at EVGA's booth, and got to go around the con after my shift there, I saw a demo side by side gsync/non-gsync and was VERY impressed.
Next monitor I bought was GSync, and unless this one dies and I simply cannot afford another for some reason, I do not see myself going back.

Id go for gsync

 
Moved to General Software.

Depends on the game. For fpsers/mobas where you can get a consistently high fps then I'd care more about having strobing instead but for really demanding games and settings (think 30-100fps) gsync is a godsend.

Jbj5000

kram36

Jbj5000
I do quite a lot of sim racing, however i have not had a problem winning races on my 1440p 60hz monitor with a 1070. Thank all of you I will be going with the Samsung cf791

Nice. You do know Free Sync doesn't work on nVidia cards?

Yeah I know it doesn't work on Nvidia cards but I have had no stutter on my 1440p 60hz monitor on a 1070 when I enable Vsync, So I will just get the samsung and use vsync, it is a $400 dollar savings from the X34 and I get Quantum dot tech.

vsync is garbage and causes input lag. 60FPS for an FPS game is barely playable.
 
 

Not needed

NVIDIA G-Sync explained - Synchronizing Monitor and Graphics card
 
excerpt
 
"Summed up :  NVIDIA G-SYNC is a solution that pretty much eliminates screen tearing, VSync input lag, and stutter. You need a G-SYNC module into monitors, allowing G-SYNC to synchronize the monitor to the output of the GPU, instead of the GPU to the monitor, resulting in a tear-free, faster, smoother experience."
 
 From Nvidia website   NVIDIA G-SYNC™
 
excerpt
 
 "NVIDIA G-SYNC™ gives you more of what you want in a gaming experience. Incredibly smooth, tear-free gameplay at refresh rates up to 240 Hz. Unique features like HDR, frequency-dependent overdrive, and ultra-low motion blur. And a powerful competitive advantage that has made G-SYNC displays go-to equipment for enthusiast gamers."