I am assuming that you are hand-holding the GND filters in front of the screw-in CPL. You would need 4x4" ND or 4x6" (4x5", if Hitech) GND filters. I don't like Cokin filters, as I have always found their colors to be "off". Lee filters are excellent as are Singh-Ray. Hitech GND filters are very good and less expensive but are 4x5" and 1.5mm thick, vs 4x6" and 2mm thick for Lee and Singh-Ray.
If you wish to use a holder system, I would suggest the Lee, with either Lee, Singh-Ray or Hitech filters (as above). You would get the WA adapter ring (vs the standard ring) to attach the holder to the lens. Ideally, you should also purchase a 105mm CPL and fit it to the holder using the front thread adapter ring. On FF and my 16-35/2.8L II, with 2-slots plus the CPL, I can get down to 19mm-20mm. With 1-slot plus the CPL, or 2-slots without the CPL, I can use 16mm without vignetting.