Is the LG 75NU850B good for gaming?

It supports VRR, ALLM and HGiG through Game Optimizer, which cuts lag and smooths frame pacing. The panel tops out at 60Hz and the HDMI ports are 2.0, so it does not do 4K at 120Hz like premium gaming TVs.

Does it upscale older HD and SD content?

Yes. The α7 AI Processor 4K Gen9 runs AI Super Upscaling to sharpen lower-resolution sources toward near-4K clarity. Results depend on the source quality.

NanoCell is LG's LCD panel layer that filters light to tighten color and reduce washed-out tones at wide angles. The Nano Detail Enhancer works with the processor to adjust contrast and detail frame by frame.

What sound does it have?

It uses 20W 2.0-channel speakers with AI Sound Pro and Clear Voice Pro. That is fine for everyday viewing, but a soundbar or AV receiver will do more for movies.

Yes. It uses a VESA 300×300mm mount pattern and weighs about 22.5kg with the stand. Check that your bracket is rated for that weight and the 75-inch size.