You can use the GM-800 with a bass guitar (with four to six strings) with an installed BOSS Divided Pickup such as GK-5B, GK-3B*, etc. or equipped with a piezo pickup. In GK SETTING, select the pickup type (GK TYPE).
*When using a bass guitar with an installed GK pickup with 13-pin GK connector, such as GK-3B/2B, the GKC-AD (sold separately) is required.
For details on the settings, see "Configuring the Pickup Settings (GK SETTINGS)" in the "GM-800 Reference Manual".