Probably you did not take the image resolution into account. Gimp works in pixels not inches.
Read this first: https://www.gimp-forum.net/Thread-Image-size-in-Gimp
Then give more details of your image. If you can give the first 3 lines of Image -> Image Properties like this (do not worry about units, imperial or metric are ok) and we can sort it from there.
Read this first: https://www.gimp-forum.net/Thread-Image-size-in-Gimp
Then give more details of your image. If you can give the first 3 lines of Image -> Image Properties like this (do not worry about units, imperial or metric are ok) and we can sort it from there.