They say nowadays that you can bring any bag you want and use it. But they recommend theirs due to the beating you're going to put on it. I talked to a guy who "shredded" (his term, not mine) a supposedly high-end bag trying to train for a GRC with bricks in it. He bought a GR1 and is still using it today as his EDC.

It lasts 8-10 hours+, covers 15-20 miles+, carrying a pack with bricks in it and the entire goal is to make everyone work as a team. The rest is just the side effect from what I've read. They will get you out of your element and test what you're made of. They'll get you wet, get you tired, and push you to your limit. And then past it.

From what the founder of it says on their website, that's the whole point. Taking you to failure and beyond. Shattering what you previously thought were your physical and mental limits.

Personally I want to do the GRC as a stepping stone to a Heavy (24 hours+), then a Selection (48 hours+).