I can't say I've had any direct experience with the Earth (I don't address the Gust because I think it would be too small). But I know several people who have worn it, and the general consensus is: Decent pack, but don't over-stuff it as the suspension quickly begins to sag under load. That's where pack weight gets to be sort of a conundrum. On the one hand, weight is weight, and whether your packed pack weighs 40 pounds or 45 makes a difference. On the other hand, a light pack that begins to sag and deform because of its load actually will make a load feel heavier than the same load in a heavier, but better-suspended, pack. So while I think the GoLite packs are fine, I'm inclined to at least suggest you also consider a more "traditional" pack, then cut ounces on other gear.
My other concern is this: Lightweight packs use lightweight materials. And I would seriously wonder whether they'd be up to the many months of daily and severe use to which you'll put it. Lightweight packs also tend to take shortcuts on adjustability, as extra straps and buckles of course add ounces. That goes for the GoLite packs, as well as the Wild Things AT ($255; 2 pounds, 9 ounces).
I guess if I was embarking on an adventure similar to yours, my starting point for a pack would be Gregory's Reality-X ($245), a lightened version of the exceedingly popular Reality. The Reality-X has good capacity (4,300 cubic inches), great adjustability and a first-rate suspension. And it weighs 4 pounds, 10 ounces, about two pounds more than the Earth or AT. I submit that 30 pounds in the Reality-X, even allowing for the pack's greater weight, will feel lighter than the same load in either the Earth or AT.
Anyway, the guy who figured all this out years ago is not Ray Jardine, it's Yvon Chouinard, who founded Patagonia. Chouinard, of course, first made his name as a mountaineer. And on expeditions, where everyone has to carry a LOT, the relatively small Chouinard found himself at a great disadvantage when lugging loads. His solution: The tumpline, a strap that supports the load from the head. By taking the weight off the back and shoulders, a tumpline allows much freer breathing, which Chouinard felt was the true limiting factor when it came to hauling load. You've got many, many miles to cover, and might find you have the time to experiment with a tumpline as well as a light pack.
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