The factory set for Global is 15* with 50/50 symmetry.
Edge guide clamps are gimmicks and tend to bring as many problems as they solve.
One problem with clip on guides is that they effectively shorten the stone by at least the width of the knife.
Another problem with guides for knives that are substantially longer than the stone is wide, is that the knife needs to be reclamped, and it's not always easy to establish the same sharpening angle at a different clamping point. If you think about it you'll realize that clip ons don't automatically create a particular angle, but that the angle depends on how far the spine is from the edge where the clip is clamped.
I don't know Global clips, but if you absolutely muse use clips, at least look at the set at Razor's Edge sharpening -- the Jim Juranitch site. Juranitch may sell a few gimmicks, but he does not sell crap. I can say the same thing about Lee at Lee Valley ("no crap") with one exception. Unfortunately, that's the DMT guide they sell under their own name really isn't very good at all. Too much flex. Obviously, in my opinion, you should avoid this clamp-on under whatever name it' sold.
Free hand sharpening isn't very difficult to learn to do well -- but it does take some time and effort. It's the best way to sharpen.
If you feel that free handing is something you won't or can't do, the next best way of dealing with sharpening is a "rod guide" such as an Edge Pro, Lansky or Gatco. All of these are easier to learn than free handing. Each company makes a couple of kits that will handle your Globals. Edge Pro is by far the best, but not inexpensive. Kit 2, which is the way to go for most people, is around $200. By comparison you can get a Lansky diamond kit plus a couple of options for under $100.
Don't skimp on your whetstones. You need more than one, and quality makes a difference. At this time there is every reason to buy Japanese ceramic water stones, and no reason to buy traditional western "oil stones."
Hope this helps,
BDL
