In addition to that, Gimp has guides: vertical or horizontal lines that the cursor snaps to when you're drawing something.
Enable to rulers on the left and the top of the image window if they weren't already there. For a vertical guide: drag with the mouse from the left ruler to were you want the guide. For a horizontal rule: do the same, but drag from the top ruler. You can have as many rulers as you want.
To move a rule: select the move tool (4-sided arrow) and drag the guide.
To remove a rule: select the move tool and drag the guide to the ruler to the left or top ruler.