So, for example, you can't say 'I heard gossip that Johnny is off his rocker', because the NPC will process that as 'hi'. Or you can't say 'I heard gossip that We need the judges to leave Withmore for all our sakes." because that will process as sake.
I think any line that begins with 'I heard gossip that' should take precedence over the other things the NPCs are listening for. Otherwise, you have this weird, un-IC moment of editing your gossip and repeating it 10 or 15 times while you try to figure out what magic words you need to avoid.