I would think that since the gamma knife was 1999 and now it is 2009, perhaps you should go get a check up as while there can be after effects from radiation and that it does develop slowly, these seem like new symptoms. They should be checked out.
Can you get an MRI if you are knocked out (ie sedated)? Some docs will give you a pill to relax you will you are in there. The newer 3T machines are larger which is nice. Still, you have to wear the darth vader mask though... I concentrate on the mirror and look at my toes.
I think you need to go back to your doctor.
I have no experience with brain tumors, but I know that any radiation to the brain is likely to cause some permanent damage. Since you had gamma knife, you may have a tiny amount of damage causing your symptoms, but head tremors and dizziness can be caused by many things. If you want to avoid another mri for now, perhaps you should at least have some bloodwork done, check your electrolytes, things like that.
As you probably are aware, you really need to speak to a neurologist about your symptoms if they are a concern.
Hope you feel better soon.