It’s very difficult to draw a circle without a trail. Gaze data has some jitter which requires filtering. So you will either have noticable latency or visible jitter.
There are some possibilities that this might not be a problem in near future so stay tuned.
Default settings (50 trail, 75% filtering) should be a good compromise.
You can also try “inverse” mode with relatively big size, maximum trail and very transparent black color. In this case parts of the screen you are looking at will be slightly dimmed. But transitions will be so slow so that the user won’t really see it if he doesn’t want to.
I see the future of the project where user will be able to turn the overlay on and off on a client side or only see it during battle replays (in games like dota)/death cams (for example in counter-strike). But that will require source integration in games and/or streaming services.