I think that you're right about the guild being able to cast its net wide enough that there might be no opposed roll. However, I think you left out one of the really important uses of bluff for something like this: innuendo.
Man in a dark red cloak "The owl of Pholtus takes flight only at twilight."
Man in a dark green cloak "Yet dawn it is the time for which men hope."
Bystander "huh?"
Man in dark red cloak "I hear that the kingfisher's chick has flown the nest; the kingfisher's looking for her above the docks district."
Man in dark green cloak "The kingfisher won't find her there; is it possible she might have been washed down into the sewers?"
Man in dark red cloak "It is possible for even the lesser birds are rarely seen there. However, a group of hunters has set snares there; if the chick were washed down there, she would have to choose her path carefully and trust to luck. If she made it through she'd find herself outside of her father's territory."
Now parts of that might be a bit obvious (especially the password/countersign) but, on the whole, it's not clear what the two men are talking about. It could be the king's stolen ruby coronet or Faberge egg. It could be the kidnapped princess. Or it could be something else. Either way, it's unlikely that bystanders would understand what was being discussed. If people don't know it, gather information can't find out about it. If the PCs use summoned earth elementals to carve the secret tunnel out of their castle and never use it in the sight of others, no amount of gathering information in the brothels and bars where the workmen hang out will reveal its existence or location.
Al said:
True enough. If the Guild casts its net wide enough, there is no 'opposed roll'- it can simply Gather Info with impunity. Nevertheless, the answer I gave was to the question: What opposes Gather Info? Perhaps a more appropriate question would be "What, if anything, opposes Gather Info?"