Difference from BTW and GTG response time?

What is it?

Also, is there a big difference in terms of performance from 5 to 2 ms response time?

BTW = Black to White

This 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

This 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.

Difference between 5ms GTG and 2ms GTG? Generally no, lower generally means faster and less chances of ghosting effects, but I've seen ghosting on a few 2ms GTG monitors, but not a few 5ms. It depends on if the advertising is true and more importantly how your brain processes the information your eyes transmit to it.

Prad.de says about ghosting or blur:

"The only negative effect the [monitor XY] demonstrates is motion blur (slight blurring, e.g. in fast turns).

However, this behaviour is caused by the technology used in LCD monitors; the motion blur arises in the eye of the beholder (Sample and Hold Effect) and is experienced to a different degree by each individual."

So it seems the ghosting is not avoidable.

As you have already seen, a monitor cannot be judged on its specs alone.
You need opinions / experience of the particular models you are interested in order to make a sound purchasing decision.