Why do I need a gaming monitor? Because PC Games do not output a steady frame rate, hence why everyone in PC Gaming is obsessed about frames per second (fps). Your fps changes from moment to moment in gameplay depending on the demand the game is placing on the PC hardware, and it is that fps fluctuation and the total number of fps being produced that causes you to need a gaming monitor.
If you have a powerful Gaming PC, you will want a Gaming Monitor with a high refresh rate, 120Hz, 144Hz or 240Hz. Because, that is the number of times it refreshes per second, so it is the cap on the number of frames the gaming monitor can display.
For any gaming monitor, you will want a 1ms response time. So the gaming monitor can actually show all the frames in real-time on the screen.
Finally, if you want your monitor not to tear (show parts of two separate frames at the same time) you will want either FreeSync if you have Radeon graphics or G-Sync if you have NVIDIA graphics.