I am very worried about the symptoms you are describing for an 11 year old pet.
First, if I were you....
1. Stop the solid gold diet. It is a holistic food and I cannot even tell you the countless numbers of pets that I have seen that have issues tolerating foods like Blue Buffalo and Solid Gold.
2. Stop the Yogurt
3. Go into a vet's office and pic-up some Science Diet I/D or Iams (Eukanuba) "low-residue diet"
Both are bland and very easy on the digest tract.
4. At your pet's age, I would really push for at least a basic blood test and fecal test for an overall health assessment. I am sure you understand I must recommend that for your pet's protection since I can't do an examination.
5. Pepcid is wonderful.
Here is some more info about the dosing of Pepcid, as well as, some other drugs like Pepto.
Pepcid comes in 10mg or 20mg tablets. You can give it every 12 hours.
Pepto-Bismol (NOT TO BE USED IN CATS)
Also, for gas, I am not against you giving a capsule of "Gas-X" or the generic simethicone you can get at the local store.
That's about the most I can offer at this point.