What is GTG?

I'm not 100% sure but I believe GTG is the gray to gray response time whilst other have different response times. So if a monitor is advertised with 5ms gtg it may have a higher practical response time. I think GTG is typically faster although I'm not sure what practical difference their is.

I think the other type is back to black or something, which measures how long it takes for the pixel to go from black to white to black again, which I think is more practical.

Zellix is on the mark...

GTG is the time it take a pixel on the panel to change from GREY to WHITE then back to GREY. This time is typically a shorter time than other scales, hence why they use it ... it a marketing thing.

Some manufacturers will still list BTB which is BLACK to WHITE to BLACK typically longer time.

Anything with 8ms GTG or below will be low enough for games and movies.