That’s easy, I just tried it and I haven’t used GIMP that much in total and not at all in the previous year and a half.
You can draw a box with the paintbrush tool. 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. And I figured this out very quickly as a user not very well versed in GIMP.
As I also wrote in this comment; GIMP is meant to be an Image Manipulation Program, not a drawing program. You generally don’t use a screwdriver to drive nails into wood, you’ve got a hammer for that. Sure, you can use a screwdriver for it in a pinch, but it’s not going to do it well. Use the tools most appropriate for the thing you’re actually trying to do.