I just performed the gate valve mod to my ASM G3 earlier, though I made a few changes.
Rather than use 1-1/4" plumbing (which is extreme overkill IMO), I used a 1-1/4" to 1" reducer bushing (the 'flush' kind that fits in a coupling), which fit right into the riser tube outlet, then ran 1" plumbing through the rest of the mod.
I still have the gate valve closed about halfway, though I may have to open it back up some once the new PVC breaks in and the skimmer stops freaking out.
Here's a pic of it installed:
I also put the gate valve between the tee and elbow, so that it adjusts more like an AquaC skimmer... the riser pipe goes to the top of the riser tube inside the skimmate cup. I might cut this down a bit so that rather than have the skimmer cup overflow, it instead flows out of the drain riser, but I'm still up in the air whether or not I'm going to do it that way.
As an aside, I couldn't believe the size difference between 1" and 1-1/4" gate valves... the 1-1/4" is HUGE... almost twice the size for another 1/4" of pipe diameter. Oh, and I also found my gate valve at OSH... neither HD or Lowe's seem to believe that anybody could have a use for one of these.
