I spray most of my thread with a pure silicon. It conditions the thread and I have less lint and breakage. It also lubricates all of the thread guides as it goes through them. I recently saw a video, where they used a liquid silicon and will have to look that up.

Check your top tension also. Any small burr on the needle plate or needle will cause thread to shred.
Sometimes I use invisible thread, when I use it I also change out my needle guides, as the invisible thread makes a groove that will shred thread also. I hope this helps.

Some threads, like Glide, need very loose tension. Before you play with the rotary tension, do your adjustments on the intermittent tension. Go to YouTube, there are a ton of videos about Gammill. If none of that helps, call the factory.

What brand thread are you using?

have you checked that your needle is in the right way around the long groove should be facing you - other that this maybe the thread is caught up on something I would try re-thread the machine and try again - it will be something simple like this.
Good luck

I just purchased a Gammill Classic. I have already quilted 5 quilts. Everything has been going great. However, I have changed my thread, and needle, I've checked the bobbin thread tension, I've played with the regular tension. . . In other words I've checked everything, done everything I can think of, tutorials, the whole thing but I still have thread breakage. I bring up my bobbin thread, go to baste around the edge of the quilt and within the first stitch, the top thread breaks. Got any solutions?