Guilds will not be paired by size, so rather useless.An actual GBG idea/suggestion...
Would having an opt-in option for GBG be of benefit? Have a check box somewhere for each player, which they must select prior to a round of GBG to be included in it (which would stay at whatever a person selected until she/he changes it). That way a guild will more accurately be paired with other guilds with regards to size. And, thus, guilds won't be penalized for not having all members wanting to participate in it.