The standard advice is to wait to get tyour pack until you have the rest of your gear. In other words, first get your shelter, your sleeping bag, your pad, your cooking gear, the clothing you will carry (as opposed to wearing) and your "miscellaneous." There's a really good reason for this--you really don't know what pack volume or structure you need until you have the stuff that's going in it! By getting the pack last, you'll be sure the pack and gear fit each other, that the pack will support the load and (most important!) that the fully loaded pack is comfortable for you. Pack fit is almost as individual as shoe fit!

While the Gorilla has a frame, the other two you list are frameless. Unless you are sure your total pack weight (that includes food and water) will be under 20 lbs., you probably want a pack with some kind of frame, such as removable stays that you'll want to keep in. I say "probably" because indivduals vary as to how much they can carry in a frameless pack. Some can do 25 lbs and others become uncomfortable with more than 15 or even less.

If overloaded, any pack is going to buckle, leaving most of the weight on your shoulders rather than on your hips. This can happen with framed packs, too. That's why it's important to try the pack on and walk around for an hour or two with the exact gear you're going to carry, plus the equivalent in weight and bulk of 5-7 day's food and a liter or two of water. You want to be sure the fully loaded pack is suports the load and is still comfortable after several hours of hiking. If it's a nice day, hike around your neighborhood, making sure to keep the pack clean so you can return it. I had to do my trial hike around the house (boring!) because it was pouring rain the day it arrived and of course I didn't want to wait. If you order a pack, be sure to check the return policy of each firm so you can return the pack if it doesn't work for you.

You can find lightweight framed packs with removable stays weighing 1.5 to 2 lbs. Examples (not all-inclusive):

ULA Ohm 2 (or their Circuit, but that's a bit heavier)
Gossamer Gear Mariposa or Gorilla
Six Moon Designs Starlite (a bit larger)
Elemental Horizons Aquilo or Kalais
Hyperlite Mountain Gear packs
ZPacks Arc Blast (this one is really light, despite the frame, but it's also pretty small)

I'm not familiar with all of these but understand most of them will support up to 30 lbs. I know the Starlite will support up to 35 (I have its discontinued "little brother," the Comet, and carried 37 lbs. in it once).

Lots of articles on gear selection on the home page of this site, left hand column, just in case you want more information overload!

For the tent, you might want to look at lighter tents from such makers as Tarptent, Six Moon Designs and Lightheart Gear. You should be able to find one under two pounds. All three makers have some essentially double-wall tents, if that's what you prefer (solo versions are the Tarptent Notch, Six Moons Skyscape, Lightheart Gear Solo or Solong (the last for tall people).

I personally don't go for sleeping bag liners. I tried one once, for three whole nights. Each morning, I was so tangled up in it that it took me several minutes to get untangled and out of the sleeping bag. After three nights, I ditched the thing. I didn't notice any temperature difference, either. Your mileage, of course, may vary. I wouldn't count on a liner to get that Summerlite down to 20*. Plenty of warm clothing worn inside should work, though.

The Exped UL7 pads are considerably lighter than the ones with the pump. You can get what they call a "Schnozzle" bag to blow it up (it comes with the UL7 Downmat but not the Synmat). I haven't tried these but have read lots of good things. A lot of people love the NeoAir, although those who don't like it hate it--nobody seems to be lukewarm about it!

Hopefully our BrianLe will come on here to give you more specific AT advice; he is a Triple Crown hiker. He just wrote a book on preparing for a long distance hike--look at the Backcountry Books section.