The dreaded knock sensor code....
A mate had that in his '99 GC8 and ended up, or I ended up, having to rebuild the engine due to excessive big end clearance.. so bad that all of the rods needed to be resized..
I had the same with my '95 GF8 after doing heads on it, and after putting a globe in the CEL. I replaced the knock sensor and it hasn't come back since..