Here's a fairly comprehensive list of gimbal heads, sorted by types...
http://www.carolinawildphoto.com/gimbal_list.htmWhat you won't see on that list are most of the cheap Chinese knock-offs, probably made by one or two factories, but selling under a bunch of different brand names: Meike, Neewer, Beike, Koolhaoda, Mcoplus, Shoot... and, yes, Movo and Sevenak.... as well as many others. Who knows the build, quality control and materials used in those. Opteka GH-1 is on the list and appears to be one of the lowest weight-rated.
You didn't mention what tripod and head you already have. Nor do you indicate which of the Sigma 150-600mm you are using.... if memory serves the Contemporary weighs about 4.5 lb., while the Sport version weighs about 6 lb.
IF you have a sturdy tripod with a heavy duty ballhead (50 lb. rated or higher ideal.... 30 lb. rated minimum, I would thing), you might want to consider using one of the gimbal adapters instead. Those work in conjunction with the ballhead, which remains on the tripod. This arrangement allows you to quickly and easily switch the tripod back and forth for gimbal versus regular use. All the other types of gimbals completely replace any existing head on the tripod and make it a sort of "long lens only" rig... not usable with camera and shorter lenses without first swapping out the heads (which may require tools).
The Wimberley Sidekick (SK-100) and Induro GBHA are two similar style gimbal adapters that are heavy duty enough to handle most long lenses. When I bought mine, the SK-100 was recommended for up to 500mm f/4 lenses like I use... which weighs about 8 lb. The Jobu BWG Micro adapter is less expensive, smaller and lighter but claims somewhat lower weight capacity. Jobu rates it for up to a 400mm f/4 DO lens weighing around 5 lb. with it's lens hood. If you use the Sigma Contemporary lens, the Jobu adapter might be an option. But if you use the Sport, it may not be.
HOWEVER, I've often seen folks using the Sidekick without problems for lenses considerably bigger and heavier than recommended. So there may be some latitude... though it's at the owner's risk, no doubt. Full size gimbals (J-post or U-post) with horizontal mounting platform are usually recommended for the heaviest lenses (which yours is not).
Your lens also is not internal focusing/zooming. Because of that, the equilibrium of the lens & camera on the gimbal will change slightly as you zoom. It's not ideal, but certainly doable (I use a similarly variable length Canon 100-400mm II on gimbal a lot). I know the Nest gimbal from Camera Cottage has been designed to help deal users with this, thoug. I am not aware of the details or any other gimbal heads or adapters taking this into consideration. On both the Sidekick and a cheap, Chinese knockoff full size gimbal I use on another tripod, my IF/IZ 300mm and 500mm lenses stay nicely balanced, but my 100-400mm II doesn't so when I'm not holding it in place I have to lock it down more carefully to keep it from tipping.
If you go the ballhead/gimbal adapter route, the ballhead MUST have an Arca-compatible quick release platform to work with the adapter. With any gimbal head you'll also need an Arca-style lens plate for use on the lens' tripod mounting ring foot (sometimes provided with the gimbal... an oversize plate allows one to adjust the balance.... plate also should have anti-twist features). With the adapters, you'll likely want Arca-style camera plate(s) so that you can most easily work with shorter lenses. Another benefit of the side-mount adapters (and full size gimbals) is that you can use them to directly mount camera in vertical/portrait orientation too, when using shorter lenses with it. That makes a bulky, expensive L-bracket on the camera largely unnecessary.
Can't compare the three gimbals you mention... They're probably all made in the same factory by the same people using the same materials.... just have different names slapped on them.