How to draw a line in Gimp is rather easy, but not apparent from the menu. First you want to make sure your foreground color is selected and you have selected a brush. You can select a brush by double clicking the current brush on the main dialog, or by opening the brushes dialog.  Now select the tool you want to use, such as the pencil, brush, airbrush, or eraser..

Then left click and release on the starting point of the line in the image.  Move your cursor to the end point of the line and press the shift button and you will see a preview of the lines path.  If this is what you want then left click to draw the line.