This is extremely discouraging. For a moment I thought that I should give Gimp a try, but not being able to draw vector shapes is a deal breaker.

The PS shape tool provides the building blocks for designing interfaces. Shapes can be scaled and merged; you can edit the vector paths and use them as masks; and much more.

Interface design requires only a fraction of Photoshop's functionality, but that is what is missing from Gimp. For image editing it might be a decent choice, but not this.