The G-Clip is very adjustable and works with or without a belt, but has a few downsides. It requires a sheath with eyelets placed to fit it, I don't remember the distance between eyelets. Also, there is the possibility of the clip interfering with drawing/resheathing the knife as it fits very closely to the sheath and can "pinch" the mouth of the sheath if mounted too high. This has happened to one or two sheaths I owned, and I ended up switching my IWB sheath attachment to the Blade-Tech IWB Loops. These are made of a highly flexible yet durable rubber and require a single eyelet to mount. Infinitely adjustable too. I have used these for the last two years with no problems at all. When set up correctly, the G-Clip works great, I just prefer the IWB loops a bit more. If you need more info I would strongly recommend contacting Mr. Mike Sastre at River City Sheaths.
http://www.rivercitysheaths.com/
Mr. Sastre worked with Spyderco to develop the G-clip and you would greatly benefit from discussing your questions with him. I have done business with him several times in the past and he is a true gentleman and a man of his word. Hope this helps