As long as you follow the instructions then I can't see any problems.
The gunson tool is a bottle with one pipe going into the brake fluid resevoir, the other end is connected to the spare wheel via the normal valve. On the clio you can just use the normal road wheel as this will be removed to bleed the brakes anyway.
Once you've connected it up you just loosen the bleed nipple and the fluid is pushed through under the pressure.
It's pretty simple and gives better results than doing it the old fashioned way, by pumping the pedal.