Since I've seen many questions on what stuff is needed to replace the head of the Gitzo Series 2 6X Traveler Kit with the Markins Q-Ball Q3 Traveler I thought I could contribute with my setup that works absolutely perfectly. For me it took many hours of reading up from different sources and a few orders to get everything right, so I hope this reply will help someone to get it right faster than me...
The original head only has one knob that controls all movement incl. how smooth it moves. I read many reviews about how bad this head was before I bought the kit, but still decided to try it out before replacing it. I have now used it for a summer and don't like it at all, so I have like many others chosen to replace it. After countless of hours of reading up I came up with either Really Right Suff's BH-30 or Markins Q3T. I selected the Q3T since it had better reviews overall, has a separate control for setting the smoothness of movement and almost perfectly fits between the legs of the tripod also when it is in "transport" position and therefor don't add any bulk. RRS also has the larger BH-40 head with a even better smoothness control knob, but it looks too big for the tripod in my opinion.
So, after deciding on the Q3T (http://www.markinsamerica.com/MA5/Q3T.php) and got it home I had my first surprise. After removing the original Gitzo head I realized that the head has a screw firmly attached and not a hole like most other heads. This of cause meant that I realized that I needed a stud to make the head being able to mount on the tripod. First I attempted to detach the stud from the Gitzo head, but to no luck. Even after dismounting it thoroughly I could not get it loose.
Next I spent hours again to try to find out what screw would be suitable... I found this thread and many others talking about parts like D01.110, D0702.01, D1550.01, GS2320D, D0701.05. I contacted different Gitzo distributors and got various replies. Finally I decided to chance a bit and ordered the Gitzo SER.2 ALUM. POWER DISC GS2320D (http://www.gitzo.com/ser2-alum-power-disc). This small kit contains a stud and a plate that makes the Q3T fit perfectly, very tight and stable, to the Gitzo legs. Even if the plate is a tiny bit larger (about 5mm) in diameter that the Q3T base, there is no problem what so ever to tilt the head to the 90 degree portrait location or to fold back the legs. The legs do not COMPLETELY fold up as with the original head, but the difference is almost not noticable, maybe a 2-3 cm in circumference, and easily fits the original bag for the Gitzo kit.
The Q3T head uses Arca-Swiss quick release plates while Gitzo uses its own, therefor I also had to replace them. Gitzo's plates are quite a bit larger than the arca-swiss. I was not that happy with the Gitzo plates anyway since they had too rough corners to keep on all the time when using the camera handheld dangling... I also wanted a L-shaped plate for the camera to simplify switching between portait and landscape without readjusting the head (and having it hanging outside the tripods mid point). After a bit of looking around I decided to order plates from Really Right Stuff. I have a Canon EOS 7D and a Canon EF 70-200mm f/2.8L IS II USM Telephoto Zoom Lens. Therefor I ordered the lense plate L84 (http://reallyrightstuff.com/Items.aspx?code=LenCan70-200f28&key=cat) and the L-plate for Canon 7D (without battery grip) - B7D-L (http://reallyrightstuff.com/ProductDesc.aspx?code=B7D-L&type=0&eq=&desc=B7D-L%3a-L-plate-for-Canon-7D-without-grip&key=it). They match the Q3T very very good and is FAR better than the Gitzo plates. Very well made, slim with smooth corners.
I.e. if you want what I think is the best tripod setup on the market for travelling, i.e. extremely lightweight but very stable tripod with one of the smoothest heads there is, go for my setup. You will not regret it!