For clarification - this is a screen dump from the dreaded Photoshop
In Ps we have ‘photoshop inches/cms’ which are NOT the same physical size as REAL WORLD inches - this is solely due to the users/viewers screen ppi and nothing else.
In this case I’m on a 27" Eizo with a screen resolution of 109 ppi but the image is 300 ppi - see the problem there??
GIMP will be the same I’m sure.
So Ps give you the facility in Preferences>Units & Rulers to basically ‘program’ Ps with your monitor ppi resolution - hence clicking View>Print Size it drops the view magnification down to the point where screen inches and real-world inches are the same!
But you don’t have to have that facility - all you need to do is divide screen horizontal ppi by image ppi then x100 and that gives you print view - in this case 36.33%.