MSAA smooths edges of objects and character models. The higher the number the smoother it gets (less jaggie outer lines). Sometimes MSAA 8 can make things blurry, I like 4 as a personal preference at 2560 x 1440.
Texture quality - turn this up your pc can handle it.
Everything that has a slider just crank it up.
SSAO - this one is taxing on ultra but makes the lighting much better.
Shadows - is kind of subjective in a way. I keep mine on high for the most part, or just a bit lower. Honestly, doesn’t affect much at all but high shadows is quite taxing.
Turn off vertical sync if you are using gsync. Useless, and will throttle your card. Wows vsync just sucks. You’ll experience frame skips if your card fluctuates with fpa at all near the monitors cap which can be frustrating. Though if you don’t hit the cap of 200hz ever, then fire away it will smooth the fps a bit.
Turn off triple buffering too, unless you have v sync on.
Turn on the multisampling option in the advanced tab, this pairs with MSAA setting.
View distance, ground clutter, and environment detail can be taxing at 10, but the 3090 should push it np.
If you have issues with these settings Turn them down one by one a bit until everything looks good and is smooth. If you have a good cpu as well none of these settings should be too taxing.
You asked about gsync. Gsync effectively matches the frames you see to the monitor. This is why it’s smooth. Have a read up on it. Turn it on in your monitor settings. There should be settings directly on your monitor with buttons or a joystick, ensure gsync is on.
Then make sure you have it on in the Nvidia control panel.
Happy gaming.