BTW, the Gamma MX is a bit heavier than the Gamma AR, which is heavier than the Gamma LT. A pant LIKE the MX (other companies make great pants) can be used for the vast majority of backcountry touring/skiing conditions, although I usually throw a lightweight pair of Gore Tex pants in my pack for a potential problem where I might have to be less active or the weather turns really bad. Having said that, I've worn the MX in in some rather windy and cold BC conditions without issue (using proper base layers) and the extra Gore Tex pants are rarely used. I also occasionally use the MX for X-country skiing, but usually wear something lighter, particularly when it's not that cold (e.g. down to -10 c (14 f )). However, lighter weight pants typically don't work as well four BC touring/skiing, and not at all for DH skiing.
I've used the MX pant for DH skiing, but they don't work as well as nylon/Gore Tex or other dedicated DH pant. Not quite as snug and warm, especially on the lift.
I don't think that your going to find a single pant that can do X-country, BC touring/skiing and DH skiing. There will be a substantial trade-off, depending on the venue. Gore Tex will be too hot and not breathable enough for XC and BC skiing. Heavier knit fabrics (like that used in the Gamma MX will work well for XC and BC, but not as well for DH.
Personally, I'd avoid buying a pant off EBAY I had not already tried on the size and model before.