Question about Bosch Professional Zubehör 30-piece milling cutter set, 6 mm shank for hand milling cutters

GKF has a 6mm holder.
The milling cutters have a 6mm shank. So these are exactly the right ones.

As far as I know, the GKF 12V-8 comes with an 8mm collet as standard, as do most newer devices. If you have an older model with 6mm, you can usually buy an 8mm collet as an accessory. This cutter set is also available in an 8mm version. If possible, I would tend to favour the 8mm cutters, as they run more smoothly and dissipate heat a little better.