GERD is very common, had it for years. Usually controllable with dietary changes and/or over the counter meds. Maybe keep a few gaviscon chewable tablets with you on the bike to use as needed.

I got the gaviscon but what dietary changes did you make?

Just Google around for the common things that cause reflux and then you can start to narrow down what is giving you trouble (most common are spicy foods, chocolate, alcohol, mint, coffees). You'll need to think about what you've eaten in the several hours before your rides, not just during the rides themselves. If I have anything with mint or very dark chocolate my stomach gets really irritated and the irritation remains long after the food has digested which screws up workouts and even sleep sometimes. If you've not previously taken medication, it will help significantly - I've been taking nexium daily for 10 years and the only things I go out of my way to avoid are minty foods and really dark chocolates.