Had a disastrous day at the Edwin B. Forsythe Nati... (
Just so you're aware, there are two or three types of gimbal heads. Basically, the first requirement to use any of them is that your telephoto lens needs to have a tripod mounting ring. And, unless one is provided with the gimbal you purchase, you also may need to purchase an extra long Arca-style quick release plate for the lens (some sort of "anti-twist" feature to that plate is pretty important). Figure $25 to $50 for that, if not included (or if you need additional for use on other lenses.) The extra long lens quick release plate makes it possible to adjust the lens & camera close to equilibrium, so that the heavy rig can be moved around smoothly with a light touch.
1. Full size gimbal head are one type and most of them are "bottom mount", designed to handle some of the heaviest of lenses such as 400mm f/2.8, 600mm f/4 and 800mm f/5.6. The mounting platform of the gimbal sits "under" the lens, sort of "cradling" it at the 6 o'clock position. There are a few full size that are "side mount", positioning the mounting plate off to one side or the other, at 3 o'clock or 9 o'clock position. Full size gimbal heads provide both vertical tilting and horizontal panning movements and completely replace the ballhead on your tripod (tools may be needed). With a full size gimbal installed, you typically won't be able to use the tripod for other purposes (short lenses with standard camera mounting). You'll first removing the gimbal head and re-installing the ballhead. But there are some accessories that allow use of the gimbal that way, without first having to remove it. Full size gimbal can be found for anywhere from $100 for a Chinese knock-off to $595 for a high quality like the original Wimberley (WH-200) to close to $1500 for some others such as the RRS Fluid head and Zenelli. Some of those prices don't seem so bad when you're trusting the head to safely support a $15,000 lens! Last time I looked, MT Shooter's carbon fiber Nest gimbal head was one of the more affordable, priced around $298.
2. Manfrotto makes a variant on the full size head that completely replaces the ballhead. Their "393" is more of a U-shape, with uprights on both side of the lens and dual locking knobs. This design can accommodate especially large, heavy lenses... but the 393 head itself is also quite heavy and pretty bulky. There may be some other manufacturers making similar, but I never had need for this type of head and don't know the details.
3. Gimbal adapters work in conjunction with a heavy duty ballhead, instead of replacing it. All gimbal adapters I know of are side-mount, as described above. In general they're intended for somewhat lighter loads, but still can handle relatively large lenses. The original Wimberley Sidekick SK-100 was rated for up to 500mm f/4 lenses weighing about 8 lb. (lens alone) and costs about $250 (I've seen folks use it with bigger lenses). The Induro GHBA is very similar and a little lower price. The least expensive is a Jobu BWG Micro that isn't offset like the other adapters and appears to have more limited capacity. With any of these you need a reasonably heavy duty ballhead, which will continue to provide the horizontal panning movement, while the adapter provides the vertical tilt movement. And, of course, any ballhead must have an Arca-compatible quick release platform to accommodate the adapter.
For more than fifteen years I've been using a Wimberley Sidekick gimbal adapter with a Kirk BH-1 ballhead (rated for 50 lb. load). Works great and allows quick swaps back and forth between the gimbal and using the ballhead without it. More recently I've gotten a full size gimbal and have it installed on another tripod.
An interesting additional use I've found with side-mount gimbals is that a camera fitted with a shorter lens and an Arca-style camera plate can be mounted in vertical/portrait orientation. This makes an expensive, bulky L-bracket unnecessary. This can't be done with the full size, bottom mount gimbals.
To allow a camera with a short lens to be used on a full size, bottom-mount gimbal, Wimberley offers an M-8 module (which also can serve as a mounting point for some of their modular flash brackets). Of course, to do this the camera must have an Arca-style plate or L-bracket installed. There may be, but I don't know of any other manufacturers offering similar. Wimberley (www.tripodhead.com
) also offers a kit that converts their full size, bottom mount WH-200 to a side mount gimbal. In fact, they have a number of accessories for gimbals and more.... including modular flash brackets, spacers and risers that can be used to fine tune lens positioning, and more.
Out in the field, a very useful accessory with any gimbal rig is a leveler. This installs between the tripod and the head. For panning movement to follow moving subjects, it's best when the head is close to level. An accessory leveler makes for much faster setups and rapid adjustments each time you move the tripod on uneven ground. While it's possible to simply adjust the tripod leg lengths, a leveler makes a much quicker job of it. There are several types of levelers from various manufacturers. For this purpose the kind of leveler that has a single locking/release lever or handle is the quickest to use. Some of those levelers are designed to be used in place of any center column. Others are designed to work either way, with a center column or not. A leveler does add some weight, but personally I think it's well worth it and have them installed on two tripods I use with gimbal heads (one full size, the other an adapter w/ballhead).
A Sigma 150-600mm C is relatively light, as telephotos go... I seem to recall it's about 4.5 lb. So just about any of the full size or gimbal adapters should be able to handle it. One thing, you lens is not "internal focusing/internal zooming".... in other words, when zoomed or focused or both it will change length. This can slightly upset equilibrium on a gimbal mount... an IF/IZ lens works best on a gimbal. However, it's not all that big a deal. Simply set up with the lens zoomed to approx. the focal length you'll be using and be a little careful to not leave the gimbal licking knobs un-tightened when you stop shooting and let go of the camera. While most of my lenses used on gimbal are IF/IZ, a Canon 100-400mm that I use isn't, is similar to your lens, and I don't have any problem using it on gimbals. I think MT Shooter's Nest gimbal has some dampening specifically to make it handle non-IF/IZ lenses better. The head and adapter I use don't.
Finally, I only use gimbals on tripods... and fairly heavy duty ones at that (Gitzo Series 3 Systematics). I don't see much need for one on a monopod (only tilt movement is needed on a monopod and that can be accomplished with a smaller, relatively inexpensive, single axis head made specifically for use on monopods).