I can't tell you why it's happening, but I can tell you I had the very same experience as both you Jubileejeff and LakeErie. What I find interesting after reading LakeErie's post, is that this does not seem to be specific to Gamma Knife.
I had my Gamma Knife in Sept. 2013 and by December 2013 my ocular motor issues started. Christmas lights really made the houses bounce around. Grocery shopping was an absolute disaster, I would get nauseous and have to stop frequently to let things "settle". Driving did not bother me unless I tried to drive on multilane highways with lots of fast moving traffic, so I simply avoided that situation or I let someone else drive. Walking was also a problem as nothing stayed still. The worst of this seemed to last roughly 6-8 months and then slowly started improving. I did vestibular exercises throughout and it is a very slow process to see improvement. My neurosurgeon felt that the AN was swelling during this time period. As the tumor reduced again in size so did my ocular motor issues. If I get overtired I can still have some very mild issues with my vision when turning my head, but nothing like what it was a year ago.
I do see a neuro opthamologist on a regular basis to watch for any optic nerve problems and so far everything is fine other than some sporadic dry spots that seem to come and go on the cornea. Hopefully you can ride this out and it will improve. One day you just notice that nothing is moving!! I would keep your Dr. informed though so they are aware of what is happening to you.