This nicely made gauge fell into my hands recently for no better reason than it looked very nicely made and deserved a good home!
The use is fairly obvious from the name written on the box 'Galtona Cutter Clearance Angle Gauge' and presumably it is / was used to check that ground relief angles were correct when making or re-sharpening Galtona cutters.
I've only come across Galtona in the context of 'inserted tooth' cutters and show a chunky example in the photo below, but exactly how and what this gauge was used for I don't know . . . .but perhaps you do ??????
Andrew