One thing about regular monitors is that they’re well-intentioned. They try to be everything to everyone, and so usually do a lot of image processing to make sure they look as best as possible in every scenario. For gaming, that’s really bad. Even worse, cheap models include fixed image processing that can’t be turned off. And remember, every process applied to the image adds a delay, because the monitor’s scaler needs to work. That delay creates input lag, or the time between your game hardware outputting a signal and your monitor displaying it for you. A bunch of processing like image sharpening, noise reduction, dynamic colors, contrast adjustment, movie mode, theater mode, and so on will only serve to slow you down. Sure, those can be great when doing design work or watching a movie, but for gaming you just need a fast, high quality panel that will let the hardware running your games express itself as accurately as possible.
If total input lag exceeds 40ms, and that’s very normal for low cost regular monitors, games become essentially unplayable. Below 25ms should be fine for most people, but faster is always welcome. You’ll know right away if a monitor is completely useless for gaming because you’ll notice a delay between hitting a key or a button and action occurring on the screen. That stuff makes gaming impossible, and is very common on cheap general purpose monitors.
Gaming monitors may have optional processing, but they’ll always feature a dedicated game mode and a PC mode. Both switch processing off, minimize delay, and maximize the freedom given to gaming hardware. Make sure your monitor supports these modes!