Gimp works with pixel dimensions, not real-world, as long as you remember that when you have inches or millimetres showing.
In the print size dialogue it shows how the image will be printed using the pixels-per-inch (ppi) parameters. It is an internal setting. Change the ppi and the size sent to the printer changes but otherwise the image remains the same.
Using the scale image dialogue the width and height values are normally linked together. You can break the link and set width and height separately. That will stretch or squash an image. The changes are reflected in the print size dialogue.
example of that: https://i.imgur.com/pJ44WTG.mp4
Are you sure that is what you require ? To change the aspect ratio of an image it is usual to crop, discarding parts of an image rather than squashing or stretching
In the print size dialogue it shows how the image will be printed using the pixels-per-inch (ppi) parameters. It is an internal setting. Change the ppi and the size sent to the printer changes but otherwise the image remains the same.
Using the scale image dialogue the width and height values are normally linked together. You can break the link and set width and height separately. That will stretch or squash an image. The changes are reflected in the print size dialogue.
example of that: https://i.imgur.com/pJ44WTG.mp4
Are you sure that is what you require ? To change the aspect ratio of an image it is usual to crop, discarding parts of an image rather than squashing or stretching