18 kg is the weight of a small child... No ordinary photography tripod can carry that! Just imagine a 5-year old toddler sitting on top of your tripod
Originally Posted by Trevor Crone
Anyway, the weight a tripod can carry is usually not so much limited by the tripod itself, but by it's tripod head. Looking at the Gitzo website (http://www.boeringa.demon.nl), I am pretty much convinced the true payload of this tripod is more in the 4 - 6 kg range (including tripod head), as Gitzo recommends this one for 300-500mm lenses.
The 18kg is probably a figure that denotes the TOTAL weight the tripod might just be able to withstand theoretically, including how much weight you can hang BELOW the tripod to strengthen / stabilize it, NOT the weight on top of the tripod.