difference between gtg and btw response time estimation of displays

BTW (Black to White) used to be the "standard" method of measuring response time. This method produces high numbers or slow times because the little tiny crystals in the LCD screen basically goes from the "off position" (black), to the "on position" (white).

GTG (Grey to Grey) method results in lower response times than BTW because one color is shifting to another (other than black to white). GTG is now the standard because this method results in lower response times. There are a handful of cases where the manufacturer lists both BTW and GTG times; for example: BTW 25ms, GTG 5ms.

Taken from this forum post.