Can dogs have these human meds

After giving Spike my 14 year old dog some Gaviscon dissolved in water last month because he was really messed up from bloating, it seemed to work. I had given him his heartworm meds and he had a bad reaction to it for the second time. I have since asked my Vet if I can switch him to something less strong than Interceptor and am going to try Heartgard Plus this time. I just gave him a quarter of a tablet since he is only 18 lbs.
Anyway, the Gaviscon really seemed to do the trick. I have not used it since because he has not shown any bad signs to call for stomache meds.
I wanted to give him Pepto, but it must have gotten in the back of the fridge and frozen and it separated. I didn't want to give it to him since I thought it would be ineffective.
I also read that they can take Zantac a acid reducers. Is that correct?
I don't want to put him on something permanently, but he has stomach trouble more now that he is older. I would like to be able to help him when he needs it, rather than have to wait until the Vet is available.