Quote:I'm using GIMP on Windows and was wondering if there was a way to keep the guidelines fixed during the image rotation stage.
I have noticed that when I rotate an image, the guidelines move with the image and do not stay fixed.
Correct. You are rotating the view, not the image. No pixels are translated (moved) in the image. Export the view and it is the orientation or the original.
Quote:I also tried to make a new layer with lines on a transparent background but the lines of the layer disappear while rotating the other layer.
You can do this using paths. Draw the path(s) in make the path(s) visible and rotate the canvas, This time the image pixels are moved and you will get parts of the image cropped off.
Quote:I needed it to better rotate the image in order to make an object completely horizontal (or vertical).
For straightening an image, all you need to do is use the measure tool. Draw a line along the horizontal or vertical then apply straighten. The Gimp docs: https://docs.gimp.org/2.10/en/gimp-tool-measure.html or my 15 second animation https://i.imgur.com/opPTntl.mp4