You can draw a box with the paintbrush tool, though. That also fixes your thing about triangle. Or if you want the lines to be totally straight, use the Paths tool, then when you’re done marking the lines you want (with or without curves) you click “stroke path” and get a window to select how you want the stroke to be.
That’s either selecting the paintbrush and drawing directly (1 click and drawing) or selecting the paths tool, making the path, and choosing the line style (1 click + however many points needed + 1 click + selecting parameters (I just went for the default to test) + 1 click to confirm).
But then again; GIMP isn’t meant to be a drawing program, it’s Image Manipulation Program. Use the right tools for the right things.