Marianna has already given you the most informed response you're likely to get without consulting multiple doctors yourself. That said, I'll throw in my two cents. What I heard is that a very skilled surgeon will have no trouble working with a tumor that has already been irradiated and that the doctors who say it's problematic are simply not the best.
But with only about a 2% failure rate for GK (when performed by the best doctors), it doesn't make sense to worry about followup surgery. (I mean it doesn't make sense from a statistical standpoint. Believe me, I worried about it, too, initially and I understand your concern and laud your due diligence.) If you have chosen GK over microsurgery for all the usual reasons -- far less chance of facial paralysis and (in some cases) hearing loss, very negligible chance of chronic headaches and other complications following treatment, much shorter recovery time, much less invasive, etc. -- then why even consider choosing microsurgery now because there is a 2% chance you might have to in the future?
Please don't misunderstand me. Microsurgery is a godsend for those who need it because radiation is not viable in their case (due to tumor size or location, or because, as with Marianna, radiation failed to halt the tumor's growth) or who personally prefer it over radiation because, for example, they want the tumor completely removed and not just killed. But especially if this is your first course of treatment, it's important to realize that every form of treatment has its risks and failures.
If GK has been recommended by your doctors as a viable treatment choice for your particular tumor, and it is your preferred treatment, embrace it (I know, this is very hard to do!) and don't worry about the 2% stuff. GK is an excellent therapy with an excellent track record of success. The chances are overwhelming that it will be a successful operation for you.
Best wishes,
TW