San-Fran is one of the biggest hippy towns in the U.S. If you look hard enough you will find enough vegan-friendly/gluten-friendly places to eat!

Eliza is right about preparing.

Trying to reframe the thought of going may help. Also think about everything as a small "problem" that can be tackled one-by-one. It's easier to feel anxiety if you see it all as one huge blob of a problem because you'll get overwhelmed.

Do you have a cooler or anything like that? Pack some food for the first few days of the trip that you can eat. You could probably buy ice anywhere if you need to refill it. Then you can still eat out but feel comfortable to know you have something in the cooler. You could even stop at grocery stores if need be. Even if you don't eat most of the food in the cooler, it will lessen the anxiety because you will know its there.

Good luck! Take pictures!!!