IANAD either, and while I lack family history of cardiac problems, we are at that age where this stuff is not to be taken lightly. A lot of GPs seem to base their diagnoses off the general sedentary population. I got much better info when I switched to my current internist who is an active middle-distance runner and "gets" normal for more active folks is different. I've discussed my general situation with a good friend who happens to be a cardiologist, he also gets it (he's also a cyclist) and while I have no reason to see him now, I have a general idea of when a professional vs social visit may be recommended. As you describe it, I am a little suspicious that the GP did not discuss doing any testing when you are physically exerting yourself (unless that EKG was a stress test on stationary bike or treadmill).
If I was dis-satisfied with my doctor's assessment, I'd at least consult with another. A good physician should recognize the value of second opinions and when a patient's situation may go beyond their immediate expertise or experience. My preference is towards docs like my internist who are active in endurance sports and have a better understanding of my lifestyle, concerns, and potential risks and ailments.
AND, I would not rely on ANY medical advice you get here or at most internet forums (can't say all as I know of a few medical/illness-related forums moderated by professionals who know what they're talking about).
Ride safe!