This is what custom white balance is for. Set up your shot and put a sheet of white paper where the subject is, so that it fills the centre circle in the viewfinder and take a shot. Select custom white balance and use that image for the camera to zero the white balance on. Just make sure that the white paper is not blown to over exposure, or so dark that the image has too much noise in it. It takes five seconds, job done (see handbook).
You only need to gel the flash if you have mixed light sources, like flash in the forgound and tungsten light in the background, and need to balance them. That shouldn't be the case here. Flash should effectively be your sole light source, even if you bounce it. Shoot at max x-sync speed to ensure that the ambient light doesn't have a chance of influencing the colour and you'll be fine.
5D2, 17-40L, 50/1.8, 24-105L, 70-200L 4 IS, 580/270EX, Strato II/RF-602, Elinchroms