I'm glad that's working out for you and all but frankly the OP hasn't clarified what octave the guitars are tuned to, or at least if he did I missed that post. "GGDGDB" at first glance to me stands out as being easily achievable at G2 on regular 6 string guitar. leaving two strings right where they are, tuning one down, and tuning 3 up.
I suppose you could break out a baritone and some thick strings and have a guitar tuned that G1. but you'd think the guitarist might say something other than "Just tune to G" which to the majority of bassists out there, the lowest one we have is G1 without downtuning and modification to our setups.
I still think BEADG or DGCFA# are both doable and allow for some comfort if the guitars are tuned to G2..
but this was the OP's first post and we haven't heard back so we may never know huh?
Also this is the only thing remotely close other than this thread for a google searching of "GGDGDB tuning".. for a "Lo-Jo" .011-.032
6-string "Lo-jo"-banjo string set - heavy gauge metal for GGDGBD tuning
Barely even two pages of search results. Now DGDGBD though, known as Open G, that has plenty of information and I'd still probably play in standard tuning.
And hey..banjos are tuned to GDGBD.. these guys could be bluegrass folks for all we know.
I know a guy who commented in this thread who recently acquired a banjo, or banjitar.. *cough*