In The GIMP, use the Measure Tool to measure a length in pixels that you know the real-world length of, or of what you want it to be.
Then open Image⇒Scale Image..., use a calculator to calculate pixels / millimetres (or pixels / inches) and enter that into X resolution (and into Y resolution).
After you have pressed Scale, the image won't change, only the parameters for how it will be measured and printed.
Then change the px at the bottom of the image window to millimeters (or some other real-world unit) and use the Measure Tool to take measurements in that unit to verify that your scaling value was not off by too much. If the measurements are a bit off, open the Scale Image... box again and tweak it.
I use this method a great deal, but most often to extract dimension from images.
With flat (orthographic) blueprints and patterns from tape, this should be straightforward, like above.
With photographs, however, it gets trickier because of perspective, and relies therefore more on my interpretation. Then, measurements may be accurate only along one axis but not elsewhere in the image. Other times, you would use one feature as reference only for another feature at the same depth in the image. etc. etc.