Yes. The GM-800 is compatible with GK-Ready guitars as well as the GK-5/GK-5B.
To connect your GK-Ready guitar, use a GKC 13-pin cable, from guitar GK output, connected to the GK Converter GKC-AD analog-digital GK converter box, output using a BGK-15/30 TRS cable to interface to the GK TRS input of the GM-800.
Audio flow would be in the following manner:
GUITAR GK READY OUTPUT > connect using a GKC 13-PIN CABLE > GK IN: GKC-AD: GK TRS OUT > connect using a BGK-15/30 TRS Cable > TRS GK INPUT: GM-800.
See the "GKC-AD Owner's Manual" for details on how to connect.
*The connectivity for GK-ready guitars does not guarantee their performance.