While we don't yet have a dedicated case for the Boss GT-1000 CORE, one of the cases in our GLIDE line may be a good fit!
The dimensions we have on hand for the Boss GT-1000 CORE are: (6.81 x 5.31 x 2.56) inches. If that aligns with what you have, we would recommend looking at the GLIDE CASE FOR THE SSL 2 OR 2+