I love the size and construction of the Gr2, but the price is way out of my range. Are there any good alternatives under a hundred bucks?
Asking for that level of quality for under $100 is unrealistic in most cases. You can find plenty of comparable options for $200 or so, but not a lot less on the commercial market. Properly sewing and bar tacking the pack is paramount, and that is skilled labor, and therefore where the cheap packs sometimes take shortcuts that will ultimately sacrifice the overall durability of the bag. The cheapest packs I'd stack up against the GR would be a USGI MOLLE 2 Assault Pack, or a Marine Corps ILBE Assualt Pack, both of which actually can be had for under $100. What you give up with these is choice of color, as you are limited to the issued pattern(s), and also they won't be quite as well engineered for comfort, although they are pretty good considering.
Got to agree with Wolfman Zack. Keeping it under $100 is hard. You might be able to get one used. I've got a Blackhawk 3day Assault Pack I'd let go for that much. Ten years old and still in good shape. Not as nice as the GR2 though! Opening up to $200 lets you consider packs from 5.11, Blackhawk, Tactical Tailor, Eagle Industries and a bunch of others. I recently shifted to EDCing a GR1 (with a 20# ruck plate) and it is everything they say it is - tough, comfortable, and flexible. Just enough organization.
Had 5.11, maxpedition, and various military packs. Sold all when I got my first GR pack. My son has a GR1 26l. Super nice, but a bit big for EDC. I have a GR0 21l. Perfect size for EDC, and I love it for challenges. Have had, and sold several bullets. I'm not a gear snob, but you get what you pay for.