The cooling pads which you can purchase that are powered by a USB cable are very weak. I sometimes play games on my MacBook Pro Retina 13", and the cooling pad (USB powered) makes almost no difference to the internal temperature of the laptop.
Moreover, the MacBook Pro's ventilation system begins at the sides of your laptop, and spits warm air out of the back, in front of your screen. The cooling pads blow air to the bottom of your laptop, which does nothing whatsoever to affect the cooling system of your laptop. If anything, it will keep the laptop base cool, but that does not affect the internal temperature.
With this said, the low power of the fans and the fact that the ventilation system is completely different to that a cooling pad provides, I do not recommend using one. A good way to cool it down slightly would be, as suggested above, to control the ambient temperature of your room with air conditioning. Alternatively, if AirCon is unavailable to you, use a simple desk fan and point it towards your laptop.
I would also recommend ensuring that the surface on which your laptop is placed, before playing any games, is wiped clean and free of dust. The increased RPM of the fans means a stronger flow of air coming through the side-vents. If it picks up dust particles, they could become trapped in the mesh / internals of the laptop, slowing down the internal cooling system.
Lastly, before playing a game, be sure to free up as much CPU as possible. Close additional programs that could use up extra CPU and memory, thus causing your laptop to heat a little more. This will put less pressure on your CPU and memory. A good method is to restart your laptop before you begin to play the game, restricting the boot programs to the bare essentials, so that you have a fresh run.
I hope this helped.