You cannot connect your guitar to the GM-800 using a guitar cable. When connecting your guitar to the GK IN jack of GM-800, be sure to use the dedicated serial GK cable (BGK-30, BGK-15), which is sold separately.
*When using a guitar/bass installed a GK pickup with 13-pin GK connector, such as GK-3/3B/2A/2B, or using the Roland V-Guitar GC-1, the GKC-AD (sold separately) is required.