LED means the backlight of the monitor is done by LED lights. On older LCD screen, there are tiny crystals that change color to show the color on the screen and then 1 or 2 fluorescent tubes light up at the top and bottom and direct light through the back of those crystals to light it up. Sucks up more power, more prone to have the invertor die, etc. LED is the future and I would only buy an LED monitor nowadays.
A gaming monitor CAN mean a monitor with a low response time, usually less than 5ms and sometimes as low as 1-2ms. It may include a higher refresh rate which some people like for gaming. A gaming monitor CAN also be some hype a company slaps on a screen to sell it better, with it having no differences or advantages over any other monitor. A good response time on a screen regardless of "gaming" or not is ideal for gaming, with 5ms being the upper limit.