This isn't an accessory, but wanted you to be aware of the new machine Gammill is about to launch. It is a mid sized computerized machine called Elegante. It isn't an addition to the existing machines, but rather a whole new one. So if you have a Vision 2 you can't upgrade. It requires a trade in.
So if you ever entertained the notion of having a computerized machine, not like a Statler, you might consider checking into it before purchasing the Vision 2.
Just a thought.
I second the opinion of the Gam guide ruler. It isn't very user friendly. I had it a a conference of Gammill users recently. The concensus was the same. No good.
Get lots of bobbins and a stand alone winder. The onboard one can only be successfully used while stitching in Constant mode.
A lot of needles would come in handy as well.