Ok, here's what I can offer to the pool of information on Gyro and/or Oryg upper cables. Like I said, maybe it'll help someone else out. I found it to be a pain in the neck, but I learned a bit myself (I'll tell you just what later).
First, a pic of the four different cables I had on-hand to measure, etc.
From top to bottom, the cables I will heretofore refer to as there letters, designated here:
A: Gyro 390mm 15 1/4"
B: Oryg 430mm 17"
C: Gyro 465mm 18 1/4" (Edit *after reading Raul's post, I re-measured twice. I have an old cable and the size may be obsolete.)
D: Gyro 495mm 19 3/4" (OldSchool)
(Note: all measurements are approximated to the best of my ability)

And here's how they fit on my old Haro kneesavers, with the stem slammed all the way down. Also consider that roughly 1/2" has been cut off each bar end, bring the lever that much closer to the detangler.
Cable A: (Gyro 390mm)

Notice that it just barely fits. It might not if the bars weren't chopped.
Cable B: (Oryg 430mm)

This one fits perfectly. Oryg, not Gyro. The only thing is, the inner cable is a bit too long. I've got to have all four barrel adjusters practically maxed-out to remove the slack. That sucks. Gyro cables A & C don't have hardly any slack to be taken out with the adjusters. That's nice.
Cable C: (Gyro 465mm)

As mentioned before, the Gyro cables were cool because all the barrel adjusters are threaded in and it works great. Not too terribly long, but still hovering just above the crossbar, and I'm not crazy about that.
Cable D: (Gyro 495mm)

Oldschool at its finest. Barhops and surfers are not an issue because you can easily fit your entire leg through the loop above the crossbar, LOL. This is probably the length known as the "Poser" model these days. I couldn't hook this one up 100% because my top Gyro plate doesn't have the slits in the side, and the old "beads" at the end of the cable won't fit through. This one has such a tight inner cable that I couldn't even get it on the lever at first. That stupid plastic barrel-adjuster tightener (seen in pic A & C) was too wide. So I changed it to a slimmer aluminum one seen in pics B & D.
Yes, folks, that's the pegless T/A I built up at the beginning of the summer to shred on. Despite my best intentions to keep it oldschool, I had to put on a Gyro. My foot got caught up in the rear cable on a decade and I almost did a grand faceplant. I'm lucky it wasn't a Rolaid. Also, it was fun, quaint, and novel to do a little barspin after each tailwhip or boomerang or what have you (ala Eddie Fiola) but it got old real fast. So, on the Gyro went. Hell, it a pegless '83 T/A w/ mags. Still plenty oldschool at heart.
Anyhow.
So what did I learn? Well, it was about time I finally switched out those black plastic barrel adjuster-tighteners to the aluminum ones on my ride. Those plastic ones can't be properly tightened down without stripping anyway. Glad to have gotten that out of the way. Also, on the new ride I'm building, I suspect the bars are a bit larger and I won't be chopping them down. So cable C might be the way to go. No excessive inner-cable slack, and it might meet the crossbar in the ideal place. If not, I'm going to invest in some knarps and do some trimming so I get it just how I want. No comprimising necessary. Maybe even get a Odyssey GTX-R that has knarps built into the bearings (I don't know, though, that could be a pain with front-end maintenance).
(additional edit after reading Raul's post* I'm going to order a Gryo in 425mm. Perfect length I'll bet, and if it's like the other Gyro cables I have [as opposed to the Oryg], it won't have too much inner slack.)
One other silly thing I learned. What with the emoticons and all, if I try to write a list in my usual fashion, it comes out like this:
A)

C)
D)
HA!