Different monitor brands measure the response time differently (in terms of how many shades of grey they test between), and possibly use different formulas to distill it down to a single number, making it a mostly useless specification for comparing monitors, except in the case of monitors that list it as being 15 ms or higher, which are not suited for gaming. If you can find some reviews of the monitors you're considering that actually show the different GTG response times, including RTC overshoot percentages and RTC overshoot recovery times, that may help you compare them better.