Yes the smile makes sense because you are, in essence, moving the machine along both the x and y axis at the same time when you are trying to quilt a circle or semi circle hence that feeling the machine is fighting and not wanting to go in that direction. So check those carriage rails specifically in the areas where the wheels are when you machine head is in the 6" spot that is causing you issues.
Out of level can also cause this but not as much to the extreme. A quick and dirty level test besides using a level is take your machine head and position it just about where you have the issue. Let go with needle up. Is it ever so slowly rolling off of that spot (usually would roll along the Y access or front to back) or is it staying put? If ever so slowly rolling, even just a fraction of an inch your rack is out of level.
On Gammills you have to raise your take up roller so it is possible that may also be out of whack and causing drag on your machine head but just at that one spot along the entire length of the rack. I don't know Gammills that well so you may want to join one of the many gammill user groups. There may even be a FB group you can join. Many knowledgeable people can offer advice specific to your setup. I am confident the issue is with your rack, not your machine head.