4K or OLED??
That was my choice recently. I went for a 55" LG OLED curved job.
Couple of reasons. 1) Content. No easily available content to view. By easily available I meant I switch on my TV and either power up my PS4, Blu-Ray or Sky Box. 2) Viewing distance.
This 2nd point is the most important point that most people miss when buying a new TV. I looked on loads of technical forums and websites and did numerous calculations that showed I needed a 102" 4K TV to be able to see the difference between 4K and "regular" Blur-Ray at the distance which I sit from the TV. I've got a reasonable large living room, so where I sit is at least 3m from the screen. Even then it wasn't far enough away.
As for gaming on it? I've attached my PS4 to it and it looks so much better than either my S7 samsung or even my very old Pioneer 436SX plasma that I bought back on 2006.