There is a new 1/4 inch jack on that GM-800 for GK in! So I'm not sure what that is all about.
Looks like the new GK5 uses different cabling. So maybe no 13pin cable to your guitar? But then I'm not sure if anyone makes a built in hex pickup with that cabling besides Boss/Roland. That means you are back to an ugly wart stuck on your guitar.
https://www.boss.info/us/products/gk-5/
Edit: Looks like if you have a 13pin guitar (like one of the Godins above) then you also need one of these to work with the new 1/4 inch serial cable for the GM-800.
https://www.boss.info/us/products/gkc-ad/
I wish they still made something similar to this:
https://www.roland.com/us/products/gi-20/
That is what I have. It has no built in sounds - just a 13pin input, direct guitar out and midi in/out. It is perfect for the studio and tracks very fast (maybe since it isn't bogged down by sound engine?). I can use it with any VST instrument so the sounds available are only limited by your VST library.
It was only ~$200 brand new. They are on reverb now for a little over $200.