Hi Pam.
Your dad is/was in my stomping grounds. My fam still in Northside of Atlanta and I did my high school/some college there!  Nothin' like a little hotlanta to make your day!
Ok, at Emory, we know he was in terrific care. Now, again, this is only my opinion and others may chime in with something different but here goes.
he had his treatment in 2003, so he's 3 yrs out. Since he had GammaKnife, we do know that there is a possibility that accuracy can be a major factor. We dont' know if the metal headframe was calibrated just prior to his treatment. Also, as GK is a one-dose process, it's a hit or miss... so, not sure if the radiation hit is well enough (the core) to zap that sucker.
Now, he may be eligable for Cyberknife up at Kennestone Hospital (my sister, who does clinical trials in Atlanta, told me Kennestone just got their CK unit). Since CK is fractionated, it would be easier on healthy tissue around the growth and easier on your dad.  If he/the family have the mindset of "at this point, just get the darn thing out of my head", then.. there are many surgical processes available in Atlanta (yes, at Emory, as one) or around the country.
I don't know if your dad has visited a GI specialist re: his stomach. I, for one, would doubt that the GK or AN would have any impact on his stomach/digestive tract.  I would consult with a GI dr to have a few tests done to see.
My understanding is that surgery is usually recommended for those that are older in order to preserve what hearing is there. I do know of some that have the attitude of "let's go non-invasive first, and if that doesn't work, then go invasive".  No decision is right or wrong, but you are doing the right thing by researching, asking, researching and asking some more.
Pam, again, this is just my 2 cents/pence and take is how you want. Maybe a bit of "food for thought", or a bit helpful, but you've certainly come to the right place.  BTW, the drinks here are watered down.. best to stick with the beer at $3 Cafe in Buckhead!
Phyllis