Inkscape is Open Source like GIMP, and like GIMP it is absolutely free.
Yes you can use layers in Inkscape, but it's not necessary for most drawings. All vector objects in Inkscape are layered above or below every other object whether or not you put them on separate layers. Even without putting things on separate layers, you can still move things in front or behind other things.
It won't be a major problem if you decide not to use layers. Layers are actually pretty insignificant when it comes to vector drawings, they don't play the same important role as they do in raster image editing software like GIMP.
The best use I have found for layers in vector imaging software is where you need to lock a layer (such as a scanned drawing) in place while you create a vector drawing above it, so you don't accidentally click on it and move it. Obviously if you have lots of objects in a drawing, layers could make it easier to find your way around and organise your drawing. Although I have personally found grouping objects to be more useful.
When you are using Inkscape, you have to forget GIMP because it is nothing like GIMP at all. Almost nothing in Inkscape is comparable to GIMP, except perhaps GIMP's paths tool.
Also tablets are not as useful in Inkscape as you might think. OK, they are fine for drawing freehand calligraphic/brush strokes but absolutely useless when it comes to editing vectors. A mouse is much more useful.