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Using Guides
I have been trying to put together a simple image that will end up 190mm in width using the Guides. I need to place vertical guides 10mm , 25mm, 30mm, and the next 7 at 17mm. I have tried to create a new image the exact length that would be required, and tried to add the guides. I start from the L/H side and all of the guides are measured from the L/H side except I add the additional length each one. When I check with the measuring tool, and when I look at the measurements on the bottom of the screen they are nothing like what I have put in. I have used, Pixels, and MM and CM, but I cannot get this one to work. I have looked for a Tutorial on this, but cannot find one to answer my questiions. This is not the first time this particular task has got me very frustrated. I have also tried using the measuring tool but this does not seem to help. 
The usual warning, Gimp works in pixels, not in millimetres. Any millimetre value has to be converted to pixels and that depends on the Image resolution.

First thing to do is open Image -> Print Size and change the drop down pixels-per-inch to pixels-per-mm Make a note of that value. Get your calculator out and use it to multiply the millimetre values to give pixel values.

Then it is just entering the pixel value into Image -> Guides -> Add Guide  for each guide.

Two minute video of that.

This is shown in Gimp 2.8, if you have updated to Gimp 2.10 It is exactly the same.
Getting accurate physical sizes in Gimp requires going over many pitfalls. See this for a start. The main difficulty is coming to grips with the idea that the physical size on your screen is usually not the size that you will get on the paper, so you need an act of faith/trust.

Otherwise, works for me... In the image below, I have set the print definition to 100px/cm so my 400x400px image is 4x4cm (or 40x40mm). Setting the measurement unit at the bottom to mm set the rulers and the Measure tool accordingly...


And switching to measurements in centimetres switches the rules and the Measure tool:


And when adding a guide the guide position uses the same units:


If you unset View ➤ Dot for dot and if your display definition is properly calibrated/determined the physical size on the screen is accurate too:

Thanks again Rich / Ofnuts
I was halfway there. I did find the Pixels/mm chart, but was not aware everything had to work in pixels. Great, I reckon I am on track.

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