Well not a true tripod head as such but an extremely worthwhile addition to add to your Gitzo tripod legs, assuming you have the right model for it to work on.
Dave alerted me to this last year and was in a couple of minds as to the perceived value of this initially. But I trusted his judgement and bought one to fit to my GT5540LS. He also mentioned it briefly here, but I thought I would just add a little more from my perspective on this fine bit of kit
Simple to fit, just undo the bolt at the head of the legs to remove the top plate and it fits straight in. Do up the bolt again and you are sorted. Fit whatever head you are using and you are ready to roll.
So what does it do for you?
Well, when you are using panning type heads such as the Wimberley or in my case the Arca Swiss with the Sidekick for my 500, to ensure that you maintain a level pan, it is essential that the head is level.
Now we have all messed about for ages, adjusting this leg up a bit, reducing that one, and by the time we have finished faffing about the light or subject have gone!!!.
Well with this little addition, you can get the position roughly right, then with a quick unscrew release of the centre column, you can rapidly level the head using the built in spirit level.
It takes longer to type those few words than do it.
I have found it a great benefit since I have been using it, and it has reduced my set up times when I get to a location, which can make the difference to getting the image or not.
It works just as well for landscapes as well, as I found out last week on Mull.
Downsides? - well if you want to operate at just above ground level, you have the short column and the additional overall height that the head will sit to contend with. So not as good in that context, but I also carry the original top plate should I need to change over, although I also use the Skimmer Pod as an alternate that Dave mentioned previously.
Pictures of the SLV head can be seen here.