I always say - quite kindly - “I hope things get better for you,” and then walk briskly off - you can always come back later when they’re gone. Sometimes people need to vent to strangers. They don’t want advice (which family/friends usually give). They just want to vent.
Lot’s of different approaches, some more passive aggressive than others.
You could say, “I’m sorry, do I know you?” Basically, “Why are you talking to me?”
When he says, “Hey, I stopped eating gluten and it has really changed my life,” you could immediately reply, “That’s great for you. Me, I’m just standing here because the bread aisle was full of pushy people trying to talk my ear off. I needed a moment of silence.”
“Hold on, the voices in my head are talking to me. [Stare off into space] Yes. No, you can’t have marshmallows. Especially not circus peanuts. No, not even Lucky Charms. I know, but they give you constipation. Yes, that’s why your poop is orange.” Just prattle away until he gives up.
Give him a glare, like “how dare you talk to me”.
“Oh, gluten-free! I was looking for the free glue.” Then wander off.
Do what I do.
Ask: “When were you diagnosed with Celiac disease?”
When – not if, when – he tells you he doesn’t have Celiac, you quickly launch into the whole, “Oh, no, you’re one of those gullible fad-diet believers who thinks gluten-free is a ‘lifestyle choice’. Your kind are actually making it harder for genuine Celiac sufferers, for whom even trace amounts of gluten can cause a reaction! Get out of my sight, you disgust me!”
You don’t even have to be a genuine Celiac sufferer to say it. Trust me, he will never bother you again.
“Have you given your heart to the Flying Spaghetti Monster? He is the answer to all of your woes!”