Hi Guest,
Since your GR is 12 years old and has epilepsy, you might want to consult your vet about this problem. It is very difficult to desensitize an animal after the fear has turned into such a bad physical reaction as it has in your dog, especially with his age. Perhaps your vet might have an idea along with a general checkup to make sure your GR isn't responding to something internal.

You're caught in a difficult position. If you force your dog outside, he'll be anxious...but if you let him get away with hiding, he'll never come out again. Perhaps you might take him on a drive and exercise him in a different area from where you live. Give him a week or so to settle down and focus on something else and then work up to your usual patterns. It's an idea.
I haven't had any luck with my terrier in recent years with anything that goes pop-pop either (cars backfiring, firecrackers, gunshots, etc..) The only thing that I've been able to do is to teach her not to go running around in the house frantically panting... she now finds her "dark quiet spot" and is tolerable until the sounds are over. I don't make any soothing sounds because to her doggy brain.... this tells her that I'm scared or I'm telling her its ok to be afraid.
If we're out on the trail, I've managed to teach her to come back to me, instead of racing home. I'll hook her back up and we'll calmly continue. I won't walk directly back home because it's a bad precedent, but we head back and she gets much calmer when we safely arrive home.
Fear is contagious. If Samma starts freaking out, she'll infect my other dog with her paranoia. They feed off of each other's emotions, just like babies. I can't get upset either because that will just encourage both of them to react. That's why its so important to keep everyone, including myself calm. My other dog has a history of seizures when stressed out...so, I completely sympathize with you and the difficulty you're experiencing.
I wish you all of the best with your Goldie. Good luck.
toodles