This tutorial shows you how to get/use measuring lines in GIMP.
This will help you LOADS with your aligning work. Objects lock onto the lines
so it makes your work look a lot neater.
1.) Create a new image
2.) Where the numbers are at the edge of your numbers
click and hold and then drag down for the line to appear
3.) Place the measuring line onto the white canvas (If you place it out of the canvas is will be removed).
4.) Use the numbers for measurement.