In a Gimp imagen you can add a guide in an exact place using "Imagen/New guide (by percent)" option (see figure below):
Add exact guides to image in Gimp.
A guide can be placed in an exact place, for example, if you want one horizontal guide in the middle of the image, you can use 50 percent. If you want to separate the vertical space by three, you can use 33, 66 percent, etc.
In this way, you can add any horizontal or vertical guide in the middle of the image. This is perfect for some designs where the space is divided in blocks..
Practical exercise: Load some image. Add the necessary guides for divide the image size in two columns and three rows.