Off the top of my head:
Have the cult leader (or a trusted lieutenant who has more ranks in the skill) make a Gather Info check of his own, with a DC equal to what the PCs rolled for their Gather Info check (or the lowest roll, if they made more than one).
DC -10 or better: "Some folks were here a while ago, asking a lotta questions."
DC -5 or better: "A couple adventurer-types were in here yesterday, asking questions about you-know-who."
DC or better: "A dwarf, an elf, and a couple humans came to the tavern yesterday, and spent an hour asking questions about the Dark One. They were trying to be discreet about it, but I spotted them."
DC +5 or better: "A male dwarf cleric, a female elf spellcaster, and a couple of male human warrior-types came to my tavern yesterday, trying to ask questions on the sly about the Dark One. They weren't very good at it. They knew his name - they even spoke it aloud! - but didn't seem to know much else about him."
DC +10 or better: as DC+5, but with complete description of the PCs (facial features, armor worn and weapons carried, etc.), and the cult leader knows all the places they asked questions in (and thus may have a vague idea of where they're located).
DC +15 or better: as DC+10, plus the PCs were tailed back to their inn room, so the cult knows exactly where they are staying. Plus, the leader has heard of any notable and/or recent deeds done by the party (slaying a dragon, looting a famous dungeon) and thus has a general idea of their power level and capabilities (They like to use cold spells, they took out a nest of mindflayers by themselves...)
Circumstance modifiers on the PCs roll depending on how discreet they are (If the players specifically say "We don't ever speak the actual name while talking to the NPCs we question", then maybe they get a +2 to +4 on their roll and the cult leader gets an equal penalty. Vice-versa if they "ask everyone we meet if they've heard of Abhoth".