BILLBAX_UK - mini GK 13 breakout cable.

[ronaldmulero]

In 2023, the Boss GKC-AD has one purpose- to provide a TRS SERIAL GK digital signal to a Boss GM-800.

Its not clear from your post you are employing a GM-800

Well, now in late 2023, I formally declare that the Boss GKC-AD has more than one purpose. In this setup, I am not employing a GM-800 at all. I only use the GKC-AD to provide power through the breakout cable to the 13-pin input on the guitar. ie. Connect the GKC-AD's female "GK In" port to the breakout cable's male 13-pin GK Preamp Power Connector that is labeled, well, it's labeled "GK Power". Don't connect anything to the GKC-AD's "Guitar out" nor its "GK out".

[Elantric]

https://www.separate-strings.co.uk/breakout%20cables%20and%20more.html