Judging from those screen tests, I think Brolin would have been alright in a George Lazenby sort of way (I'm not a big Lazenby fan, but he's certainly preferable to Craig). Brolin is stiffer and much less dangerous looking than Lazenby, but he has a much smoother delivery than Lazenby too. I think it's a good thing he wasn't cast, but he wouldn't be the disaster Craig's been. Craig is changing the character of Bond. Brolin either would have lived up to the Bond standard or been quickly ousted. Cubby wasn't trying to rewrite Bond the way Babs is now.
I'm not 100% sure Gavin would have been a total disaster either. He is pretty wooden in Psycho, but that might be because Hitchcock hated him and treated him poorly as a result. I remember Gavin's performance as Julius Caesar being one of the few real highlights of the otherwise tedious Kirk Douglas ego project better known as Spartacus.
"For a moment Bond looked up into two glittering eyes behind a narrow black mask. There was an impression of a crag-like face under a hat brim, the collar of a fawn mackintosh." - Ian Fleming, Casino Royale