My two cents is that the GB1 is a fantastic knife, just not one that ticks off the right boxes for me, it's handle heavy (beastly really) and I think that the over-built handle is a mis-match with the very thin grind of the blade... if it was made of something less resilient than M4 I'd bet we would have occaisonal threads about blade fractures. I would also lay money on it being a fantastic shaving tool, due to the blade grind, though the weight might make it a bit less agile than some. It feels like a mash-up to me, a super acute blade on a nearly indestructible handle... too large and heavy to be a gentleman's knife, too refined to be a bash-around tool. I won't sell mine, but it doesn't get much pocket time.
If you want a knife that "can take it" and is designed to be easy to maintain control of during heavy cutting tasks, the M2 is a winner. I've cut a whole stand of brush and saplings with mine and while it's no machete it did more work than most would expect from a medium-large folder with no sign of weakness... yes, the lobed liner projections can promote blisters if you use it hard for an extended time, but if you don't have tough hands, then a glove would be a good idea for a session like that and those same serrations make for noticeably better grip with a glove on. Also, despite the brick ****house of a liner lock lug on the GB1, I still feel like the Manix's CBBL might be even stronger and I'm one of those odd ducks who really likes the way the CBBL functions.
I have an early-ish sabre grind M2, an S110V M2, the KC XHP M2LW and the KC CruWear M2, so it's pretty easy to tell how I feel about them
As far as handle geometry *only* I far prefer the Manix, it locks my hand in, but does so comfortably and offers several grip options (I wear a large in most glove brands, so medium sized hands), the GB1 feels nice, but nothing lines up for my hand the way that the Manix does and a slip under load is much easier to imagine than with the Manix's deep choil and quasi-sub-hilt (that's probably not a word, but I hope you get the drift).