Our next Gitzo on the list is aimed users wanting something with greater load carrying capacity, while maintaining versatility. That means only one range, the Systematics. They all have a removable top-plate for a choice of columns, should you decide you need one. However, unless you intend to use it occasionally in the studio then it's not really needed. Gitzo make both geared and what they call 'rapid' (lockable sliding) columns for the Systematics, but you must buy the right size for your tripod, either series 3, 4 or 5.
Gitzo no longer make the superb (but seriously heavy) Series 5 Systematics in aluminum, unfortunately (get them now before they sell out: G1500 and G1505). They were the staple for studios, but unless you need that in particular, the material to go for is carbon-fibre - it's pricey but light, rigid and pretty durable. As you'll most likely want to use this on location, that means choosing the four-section over the equally impressive (but slightly more stable) three-section version.
Carbon Fibre absorbs vibration pretty well, but some lightweight designs are susceptible to lateral movement and flexing - that doesn't happen with the 5-series though it does with some of the 3-series - particularly the 4-section models which have thin tubing for the last (fourth) section. I've noticed a few professional photographers use the tripods partly extended from the bottom. This is not the way it should be used. To reduce vibration, always extend the legs from the top down (using the fourth, and thinnest stage, as a last resort).
The four section carbon-fibre GT3541LS (above) doesn't come with a column, that's extra, however the platform is used to mount a head directly to it. If you need to level a head, for video capture or panoramas, then the you can replace the platform with either a video bowl adaptor (designed specifically for 75mm or 100mm video-heads, which feature their own built-in leveling bowl) or the leveling base for panoramas.
This is the industry standard** tripod for sports and wildlife photographers using long-lenses up to 500mm, with something like the outstanding Wimberley WH-200 Head - Version II (pictured below). The GT3541LS is the long version (stands 146cm tall at its maximum, packs down to 55cm (nearly the same as the Explorer above) and can carry a load up to 18kg. It weighs just 1.72kg. The GT3541XLS is an alternative if you want a little extra height but there's quite a price premium, and it's susceptible to some flexing at the tallest setting, when it must be used with care. This tripod stands 198cm at its maximum and closes down to 70cm, quite a jump over the LS version (and may cause issues with Airline carry-on restrictions) but still weighs just 1.97kg.
Neither model is quite stable enough for the 600mm f/4 lenses though. For that you'll have to read on.