Actually the green combined with the magenta gel is from the old film days. The best way to do this, and we practice this in the Virgin Islands workshops, is with digital. First, you place your green gel over your flash, then do a custom white-balance with your camera. Very easy with Canon as you just select the image. Make sure you custom white-balance the way your camera says. With Canon I have the model hold up a white towel.
By using custom white-balance, the camera corrects to the opposite (magenta) and you get a magenta sky as the flash makes the model green, but not the sky. So basically everything that doesn't get hit by green flash, turns the opposite.
Every photographer should know six colors, red, green, blue and their exact opposites, cyan, magenta and yellow. Another technique is to use a 3/4 CTO (Rosco #3411 gel) then place it over your flash and set your white-balance manually anywhere between 3200 to 4100K, depends on your model's skin tone. This is the same as shooting through an 80A filter in film cameras. Thus, the CTO makes flash tungsten, the flash then illuminates everyone with tungsten temp light, thus what doesn't get hit by flash turns blue.
The trick to all this is overpowering the sun with flash and using your shutter-speed to control the saturation of the background color, hence if your settings are 1/200 shutter at F/16, then you make sure the flash lights your subject with F/16 output. Then to lighten the sky you drop the shutter where you like, the shutter-speed for your subject is the duration of the flash.
I hope that helps, all the best, rg sends!