I am not sure why the response time is different, but 2ms GTG or jsut 5ms in general isnt noticable. To compare a standard LCD HDTV is about 20-30ms latency, and people play video games on those.
According to Samsungs website it is 2ms So I would think that number is correct.
Contrast ratio helps with picture quality, makes the darks darker and the brights brighter. Typically higher is better. Some monitors have a dynamic contrast ratio, which is my experience is annoying, but it depends on your uses.
From the specs alone that looks to be a decent monitor.