I can't count all the /6's I've done but it's sooooo many I was bored by them. I worked where we had accounts for /6's that went into forklifts and rail car moving whatever they were. Just on /6's and Continentials I've probably set 1000 sleeves. Maybe more.
I can honestly say I've never seen a GD for a /6. Doesn't mean you can't make one. Even if you did make it, and could produce it so the average guy could buy it (I'm talking about a fixed idler 3 gear drive) you'd probably never sell enough to get the intial investment back.
I hear lots of people bad mouth gear drives, but when questioned, they've either never run them, or used a junk floating idler system like the Pete Jackson system. Those aren't as good as a chain.