So Simple They Forgot to Make it Obvious
I'm not going to lie - I used GIMP for a very long time without ever knowing how to make straight lines easily. The fact is though, that not only does GIMP have the ability to make straight lines, it has the ability to use these straight lines with many of it's complex tools.
It would make sense for one to be able to make a straight line using a the basic paint brush tool, so let's start with it.
This simple image was made entirely with the GIMP paint tool and the straight line modifier. How?
Step 1: Select the Paint Brush tool. Make a single mark on the canvas - this will be the starting point of your first line.
Step 2: Hold down the shift key and move the cursor around the canvas. You will see the "ghost" image of where the line will go. Click anywhere on the page to create the line from our starting point to there.
That's the only trick to it! The all-powerful shift key. The line will mimic the size, opacity, and shape of whichever brush you choose, so you won't end up with awkward circles with lines connecting them.