I just remembered that I've got a Gansey in progress (since 1986, IIRC). I'm using 2.5mm needles (old UK ones) and getting a gauge over double moss stitch of 28st/4in (7st/1in). The yarn label doesn't give yardage, but the website for Emu (now Wendy) yarns gives it as: 224 metres per 100g ball ~~ Needles: 2.25 - 3mm ( US 1 - 3 ) ~~ Tension: 36 rows x 28 sts.
I'm guessing that's gauge over stockinette (or tension over stocking stitch) so my gauge over double moss stitch is going to be pretty close to that. The yarns are fairly similar -- the guernsey yarn is a 5-ply so a bit heavier per yard than a 2 or 3 ply of the same grist.
It's a very firm very dense fabric, but as it's all wool it's got a decent drape to it.
I think you'll lose some of the character of the pattern if you go to too fine a yarn. Palette is pretty much double the yardage, so it's too fine. You'll end up with a sweater that won't be very warm and IIRC, warmth is one of your criteria for choosing this style of sweater.
The other thing is sizing. If you were to get 8st/in instead of 9st/in, could you knit a smaller size and still end up with a garment that fits?
Try it on US#3 needles. According to my handy-dandy multilingual needle gauge, 3mm does not translate exactly to a US size. US#2 = 2.75mm and US#3 = 3.25mm.
So, my recommendation: go with the Gloss, and if you're getting slightly over gauge on US#3 go down a size -- it's not a very fitted sweater, so a little more or less ease won't hurt :D