i'm a fan of the conti 4 seasons in 28. they measure closer to 27.
haven't spent the $$ on the GBs as I haven't seen a pair in the wild, and I only know a few people that ride them (not local).
i do plenty of 'mixed terrain' rides - gravel, dirt, crumbling pavement - haven't wanted to take a chance sight unseen on the GBs.